A Dwarf Star Produces a Major Discovery

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his artist's illustration depicts an imagined view from the surface of one of the three newfound TRAPPIST-1 alien planets. The planets have sizes and temperatures similar to those of Venus and Earth, making them the best targets yet for life beyond our solar system, scientists say. Credit: ESO/M. Kornmesser
An imagined view from the surface of one of the three newfound TRAPPIST-1 exoplanets. The planets have sizes and temperatures similar to those of Venus and Earth, making them attractive scientific targets in the search for potentially habitable planets beyond our solar system.
(ESO/M. Kornmesser)

The detection of potentially habitable exoplanets is not the big news it once was — there have been so many identified already that the novelty has faded a bit.  But that hardly means surprising and potentially breakthrough discoveries aren’t being made.  They are, and one of them was just announced Monday.

This is how the European Southern Observatory, which hosts the telescope used to make the discoveries, introduced them:

Astronomers using the TRAPPIST telescope at ESO’s La Silla Observatory have discovered three planets orbiting an ultra-cool dwarf star just 40 light-years from Earth. These worlds have sizes and temperatures similar to those of Venus and Earth and are the best targets found so far for the search for life outside the Solar System. They are the first planets ever discovered around such a tiny and dim star.

A team of astronomers led by Michaël Gillon, of the Institut d’Astrophysique et Géophysique at the University of Liège in Belgium, have used the Belgian TRAPPIST telescope to observe the star, now known as TRAPPIST-1. They found that this dim and cool star faded slightly at regular intervals, indicating that several objects were passing between the star and the Earth. Detailed analysis showed that three planets with similar sizes to the Earth were present.

The discovery has much going for it — the relative closeness of the star system, the rocky nature of the planets, that they might be in habitable zones.  But of special importance is that the host star is so physically small and puts out a sufficiently small amount of radiation that the planets — which orbit the star in only days — could potentially be habitable even though they’re so close.  The luminosity (or power) of Trappist-1 is but 0.05 percent of what’s put out by our sun.

This is a very different kind of sun-and-exoplanet system than has generally been studied.  The broad quest for an Earth-sized planet in a habitable zone has focused on stars of the size and power of our sun.  But this one is 8 percent the mass of our sun —  not that much larger than Jupiter.

“This really is a paradigm shift with regards to the planet population and the path towards finding life in the universe,” study co-author Emmanuël Jehin, an astronomer at the University of Liège, said in a statement. “So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical, but now we have not just one lonely planet around such a faint red star but a complete system of three planets!”

Our sun and the ultracool dwarf star TRAPPIST-1 to scale. The faint star has only 11% of the diameter of the sun and is much redder in colour. (ESO)
Our sun and the ultracool dwarf star TRAPPIST-1 to scale. The faint star has only 11% of the diameter of the sun and is much redder in colour. (ESO)

The TRAPPIST-1 star is very faint and was identified because a Belgian team built a telescope especially to look for stars, and exoplanets, like the ones they found.  TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) is tiny by today’s standards, but collects light at infrared wavelengths and that makes it well designed for the task.

The observations began only in September, 2015, and targeted a dwarf star well known to astronomers.  TRAPPIST spends much of its time monitoring the light from around 60 of the nearest ultracool dwarf stars and brown dwarfs (“stars” which are not quite massive enough to initiate sustained nuclear fusion in their cores), looking for evidence of planetary transits.

Because the star and planets are so relatively close, they offer an unusual opportunity to potentially characterize the atmospheres of the planets and determine what molecules are in the air.  These measurements are essential to learning whether a planet is indeed habitable (or even inhabited.)

TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) is a 60 cm telescope at La Silla devoted to the study of planetary systems and it follows two approaches: the detection and characterisation of exoplanets around other stars and the study of comets orbiting around the Sun. The robotic telescope is operated from a control room in Liège, Belgium. The project is led by the Department of Astrophysics, Geophysics and Oceanography of the University of Liège, in close collaboration with the Geneva Observatory (Switzerland). TRAPPIST is mostly funded by the Belgian Fund for Scientific Research with the participation of the Swiss National Science Foundation. The name TRAPPIST was given to the telescope to underline the Belgian origin of the project. Trappist beers are famous all around the world and most of them are Belgian.
TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) is a 60 cm telescope at La Silla devoted to the study of planetary systems and it follows two approaches: the detection and characterisation of exoplanets around other stars and the study of comets orbiting around the Sun. The robotic telescope is operated from a control room in Liège, Belgium. The project is led by the Department of Astrophysics, Geophysics and Oceanography of the University of Liège, in close collaboration with the Geneva Observatory (Switzerland). TRAPPIST is mostly funded by the Belgian Fund for Scientific Research with the participation of the Swiss National Science Foundation.

Co-author Julien de Wit, a postdoc in the Department of Earth, Atmospheric, and Planetary Sciences, said scientists will soon be able to study the planets’ atmospheric compositions quite soon.

“These planets are so close, and their star so small, we can study their atmosphere and composition, and further down the road, which is within our generation, assess if they are actually inhabited,” de Wit said. “All of these things are achievable, and within reach now. This is a jackpot for the field.”

Rory Barnes, a specialist in dwarf stars and their exoplanets at the University of Washington, agreed that the TRAPPIST-1 discovery was  both intriguing today and inviting of a lot more future study.  Indeed, he said that efforts to characterize exoplanet atmospheres will most likely focus for the next decade on the smaller stars in our galactic neighborhood — the ubiquitous M dwarfs.

“It’s just easier to find exoplanets around smaller stars because they block out a great percentage of the star’s light when they transit,” he said. “And with small stars, the planets are usually closer in, which also makes them easier to find.”

But there are also significant barriers to habitability in the TRAPPIST-1 system.  Because the planets are so close to their host star — the first has an orbit of 1.5 days, the second an orbit of 2.4 days and the third an ill-defined orbit of between 4.5 and 73 days — that means they are tidally-locked, as is our moon.  Not long ago, exoplanet scientists doubted that a planet that doesn’t rotate can be truly habitable since the extremes of hot and cold would be too great.  That view has changed with creation of models that suggest tidal locking is not necessarily fatal for habitability, but it most likely does make it more difficult to achieve.

A larger potential barriers is that the dwarf star once was quite different.  Jonathan Fortney, a University of California at Santa Clara specialist in dwarf stars and brown dwarfs (objects which are too large to be called planets and too small to be stars), focused on that stellar history:

“One thing to keep in mind is that this star was much much brighter in the past,” he said in an email. “M stars (like TRAPPIST-1) are hottest when they are young and take a long time to cool off and settle down.  Their energy comes from contraction at first.  A star like this takes 1 billion years to even settle onto the main sequence (where it starts burning hydrogen).”

Barnes also focused on the stellar evolution, which he said is always complex and pertinent when talking about dwarf stars and exoplanets.  A small dwarf star like TRAPPIST-1 — which the authors estimate is 500 million years old — would have spent a much longer time as a much hotter protostar, sending out intense heat from its formation process before it achieved fusion.  That means a planet in the star’s habitable zone now may well have been baked like Venus eons ago, Barnes said, and there is no known way to become habitable after that.

So the relatively benign conditions around TRAPPIST-1 now in terms of radiation and heat clearly have not always been present.

The study authors said — and other scientists agree — that the most likely planet in the system to be actually habitable is the one furthest out.  But the orbit of that third planet has not been well defined, as seen in the estimate that it orbits its star within somewhere between 4.5 and 73 days.

As it turns out, the follow-on Kepler mission (K2) will be observing in the area that includes TRAPPIST-1 from this coming December through March 2017.

Kepler Mission Scientist Natalie Batalha said that she hoped the team put in a proposal to observe TRAPPIST-1.  If they did, she said, the proposal will be peer reviewed this month and could be among those selected. Assuming the telescope is in good working order and operations continue to be funded come December, K2 observations could better define that third planet’s orbit.

The Trappist-1 system is at the edge of the field that will be observed starting in December. The graphic shows detector that Campaign 12 detector field. (NASA/ Natalie Batalha)
The TRAPPIST-1 system lies within the field that is planned for Campaign 12 starting in December. The graphic shows its predicted location at the edge of one of Kepler’s detectors. (NASA/ Natalie Batalha)

But whatever happens with K2, TRAPPIST-1 is now an astronomical “star” and will no doubt be getting scientific attention of all kinds.

 

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The Borderland Where Stars and Planets Meet

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Brown dwarfs -- like the one illustrated here - are more massive and hotter than planets but lack the mass required to become sizzling stars. Their atmospheres can be similar to Jupiter's, with wind-driven, planet-size clouds. (NASA/JPL-Caltech)
Brown dwarfs — like the one illustrated here – are more massive and hotter than planets but lack the mass required to become sizzling stars. Their atmospheres can be similar to Jupiter’s, with wind-driven, planet-size clouds. (NASA/JPL-Caltech)

Results from two very different papers in recent weeks have brought home one of the more challenging and intriguing aspects of large exoplanet hunting:  that some exoplanets the mass of Jupiter and above share characteristics with small, cool stars.  And as a result, telling the two apart can sometimes be a challenge.

This conclusion does not come from new discoveries per se and has been a subject of some debate for a while.  But that borderland is becoming ever more tangled as  discoveries show it to be ever more populated.

The first article in The Astrophysical Journal described the first large and long-lasting “spot” on a star, a small and relatively cool star (or perhaps “failed star”) called an L dwarf.  The feature was similar enough in size and apparent type that it was presented as a Jupiter-like giant red spot.  Our solar system’s red spot is pretty well understood and the one on star W1906+40 certainly is not.  But the parallels are nonetheless thought-provoking.

“To my mind, there are important similarities between what we found and the red spot on Jupiter,” said astronomer John Gizis of the University of Delaware, Newark.  “Both are fundamentally the result of clouds, of winds and temperature changes that create huge dust clouds.  The Jupiter storm has been going for four hundred years and this one, well we know with Hubble and Spitzer that it been there for two years, but it’s probably more.”

A far cry from 400 years, but the other similar storms and spots identified have been on brown dwarfs — failed stars that start hot and burn out over a relatively short time.  Gizis said some large storms have been detected on them but that they’re gone in a few days.

The dust and wind storm on the L dwarf W1906+40 rotates around the cool star every nine hours and is large enough to hold three Earths. L-dwarfs mark the boundary between real stars and “failed stars” only the most massive L dwarfs fuse hydrogen atoms and generate energy like our sun. Most L dwarfs known are brown dwarfs, also known as “failed stars,” because they never sustain atomic fusion. (JPL/NASA-Caltech)
The dust and wind storm on the L dwarf W1906+40 rotates around the cool star every nine hours and is large enough to hold three Earths. L-dwarfs mark the boundary between real stars and “failed stars.” . Most known L dwarfs are brown dwarfs, also known as “failed stars” because they never sustain atomic fusion, but the most massive L dwarfs can fuse hydrogen atoms and generate energy like our sun.  (JPL/NASA-Caltech)

 

The second article came from Alexandre Santerne of the Instituto de Astrofísica e Ciências do Espaço, Portugal and Aix Marseille University, France, and was shared and widely discussed at the recent Extreme Solar Systems meeting in Hawaii.  In the paper, the researchers report that a high percentage (55 percent) of the very large exoplanet “candidates” listed by the Kepler mission are in fact not exoplanets. Santerne and colleagues spent a year’s worth of nights between 2010 and 2015 observing, via the radial velocity method, 129 of Kepler’s more than 4,000 planet candidates.  Their tool was the SOPHIE spectrograph at Haute-Provence Observatory  in southeastern France.

The Kepler science team has long predicted that the “false positive” rate for these very large radii planets would be high — a projected 30-40 percent rate for candidates larger than Jupiter versus less than 10 percent false positive rate for candidates smaller than Jupiter.  But this even higher percentage came initially as something of a worrisome surprise.

Many of what the Santerne team described as “false positives” were determined to be multi-star systems (rather than a star with planets)  while three were identified as brown dwarfs, those  small, cool failed suns.

Said team member Vardan Adibekyan of the Centre for Astrophysics of the University of Porto:  “Detecting and characterizing planets is usually a very subtle and difficult task. In this work, we showed that even big, easy to detect planets are also difficult to deal with.”

While finding many false positives, the Santerne team also confirmed 45 Kepler very large planet candidates, fifteen more than had been confirmed before.

Size comparison of stellar vs substellar objects. (Credit: NASA/JPL-Caltech/UCB).
Size comparison of celestial objects from our sun to Earth. (NASA/JPL-Caltech/UCB)

Natalie Batalha, Mission Scientist for the Kepler Space Telescope mission,  said that at first glance the reported false positive rates seemed higher than expected based on predictions by the Kepler team,  in particular the modeling work of astrophysicist Timothy Morton of Princeton.

But after a careful read and some number crunching, Batalha said she came away confident that the new results do not reflect any flaws in the planet identification process itself and, in fact, agree with predictions.  The apparent rise in the false positive rate, she said, can be attributed to a more liberal inclusion of larger exoplanet “candidates” initiated in 2014 by the Kepler mission.

Previously, planet candidates more than twice the radius of Jupiter were all discarded because no planets above that line had ever been detected — they were deemed “astrophysical false positives”.   But they were returned to the “candidate” list a year ago so that scientists could explore the transition between giant planets and brown dwarfs and small stars.   Once these larger-than-two-Jupiter “candidate” planets were folded back into the Jovian planet group, Batalha says, the false positive rate for the group naturally shot up. Which is predictable, since no two-Jupiter planets were identified by the Santerne group.

Nonetheless, she said, the results reflect and illustrate the complex nature of large exoplanet detection and characterization. “The truth is that we don’t know a lot about the transition from giant planets to stars.  It’s an important subject and this team is one of the few working on it.”

Colors plotted here represent the average expected false positive rate for candidates of a given size (radius on the y-axis) and orbital period (x-axis). For candidates smaller than Jupiter, the expected false positive rate is less than 10%. For planets between one and two Jupiter radii, the false positive rate jumps up to 38%. For objects larger than twice the size of Jupiter, the false positive rate increases to 90%. White points show the properties of the candidates observed by the Santerne team. (NASA Exoplanet Archive/N. Batalha, T. Morton
Colors plotted here represent the average expected false positive rate for candidates of a given size (radius on the y-axis) and orbital period (x-axis). For candidates smaller than Jupiter, the expected false positive rate is less than 10%. For planets between one and two Jupiter radii, the false positive rate jumps up to 38%. For objects larger than twice the size of Jupiter, the false positive rate increases to 90%. White points show the properties of the candidates observed by the Santerne team. (NASA Exoplanet Archive/N. Batalha, T. Morton

 

As determined by the International Astronomical Union, any celestial object with a mass greater than 13 Jupiters should be considered a star.But according to Jonathan Fortney, an exoplanet and brown dwarf theorist at the University of California, Santa Cruz, this definition leaves a lot of researchers cold because it doesn’t take into account how the object was formed. Did it form in a giant molecular cloud (like most stars)?  Or in orbit around a parent star, by slowly adding on large amounts of gas, atop a solid core of rock and ice (like most planets)? Or as a result of gravitational instability in a disk (a theory that suggests the formation of massive gas giant planets as the result of a quick pulling together of disk material to form dense clumps)?

“It seems clear that star formation can make objects less massive than ten Jupiters and we can see planets more massive than several Jupiters in disks around stars.  So there’s an overlap here, and we don’t always know when star formation stops and planet formation starts,” Fortney said. “That why it’s so important to learn about the composition and evolution of the objects to figure out what they are.”

Artist's conception of the clouds on Kepler-7b, compared for size with Jupiter (right). Many exoplanets and brown dwarfs have mostly hydrogen-helium atmospheres that are covered in layers of mineral dust, while Jupiter’s hydrogen-helium atmosphere has clouds of ammonia. (NASA/JPL-Caltech/MIT)
Artist’s conception of the clouds on Kepler-7b, compared for size with Jupiter (right). Many exoplanets and brown dwarfs have mostly hydrogen-helium atmospheres that are covered in layers of mineral dust, while Jupiter’s hydrogen-helium atmosphere has clouds of ammonia. (NASA/JPL-Caltech/MIT)

And of particular interest in that borderland are brown dwarfs, convincingly identified only twenty years ago.

As Fortney explained, brown dwarfs are formed in the same vast clouds that produce stars by the hundreds, but don’t have sufficient mass to build the internal pressure needed to begin the nuclear fusion of hydrogen that defines a star.  Still, the gravitational energy of a brown dwarf does get converted into heat and so they can warm their surroundings before cooling like embers leaving a fire.  Some researchers even hold that planets could form around brown dwarf and protoplanetary disks have already been found around a few of them.

What particularly fascinates Fortney about brown dwarfs is that they have atmospheres and winds and weather, and as a result offer some potential insights into larger exoplanets, especially those surrounded by thick dust clouds.

This overlay of suspended minerals (sometimes exotic metals like aluminum oxide and magnesium-rich forsterite — a form of silicate rock — and irons) have made it very difficult if not impossible to look spectroscopically at the atmospheres of many exoplanet.  But depending on the temperatures and compositions of the dust clouds, astronomers sometimes have more luck  looking through the clouds and haze of brown dwarfs.

But still, the process of getting information about distant atmospheres is painstaking and Fortney said his work with brown dwarfs provides “a window into just difficult it is and will be” with exoplanets.  Basic questions like temperatures, what kinds of molecules are present and in what abundances — they’re all veiled by the dust clouds.

Cosmic dust surround a brown dwarf in the making. ALMA (ESO/NAOJ/NRAO)/M. Kornmesser
Artist’s rendering of a brown dwarf surrounded by cosmic dust and gases. ALMA (ESO/NAOJ/NRAO)/M. Kornmesser

Progress, however, is being made, both in terms of technical approaches to “seeing” through the clouds, and the science of these objects.  Even gigantic exoplanets appear to have clouds and dynamic atmospheres, Fortney said, “and I think we’ll see that across the board.”

Batalha also identified a related bit of progress.  The Santerne paper identified three brown dwarfs in the Kepler candidate list, she wrote, and so they produced the beginning of an occurrence rate for brown dwarfs. In addition, the paper published an occurrence rate for warm Jupiter-size planets within one astronomical unit or AU (roughly the distance from the sun to Earth) of their own sun.

Putting the two observations together, and you reach the conclusion that warm Jupiters are 15 times more common than brown dwarfs in similar one AU orbits.

That, she said, is the intriguing  news coming from the giant planet/failed star borderland.

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On Super-Earths, Sub-Neptunes and Some Lessons They Teach

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Part 2 of 2

The Kepler-452 system compared alongside the Kepler-186 system and our solar system. Kepler-186 is a miniature solar system that would fit entirely inside the orbit of Mercury. The size of the habitable zone of star Kepler-452, considered one of the most “Earth-like” exoplanets found so far, is nearly the same as that of our sun. “Super-Earth” Kepler-452b orbits its star once every 385 days. (NASA Ames/JPL-CalTech/R. Hurt)
The Kepler-452 system compared alongside the Kepler-186 system and our solar system. Kepler-186 is a miniature solar system that would fit entirely inside the orbit of Mercury. The size of the habitable zone of star Kepler-452, considered one of the most “Earth-like” exoplanets found so far, is nearly the same as that of our sun. “Super-Earth” Kepler-452b orbits its star once every 385 days. (NASA Ames/JPL-CalTech/R. Hurt)

 

With such a large proportion of identified exoplanets in the super-Earth to sub-Neptune class, an inescapable question arises: how conducive might they be to the origin and maintenance of life?

So little is actually know about the characteristics of these planets that are larger than Earth but smaller than Neptune (which has a radius four times greater than our planet) that few are willing to offer a strong opinion.

Nonetheless, there are some seemingly good reasons to be optimistic, about the smaller super-Earths in particular. And there are some seemingly good reasons to be pessimistic –many appear to be covered in a thick layer of hydrogen and helium gas, with a layer of sooty smog on top, and that does not sound like an hospitable environment at all.

But if twenty years of exoplanet hunting has produced any undeniable truth, it is that surprising discoveries are a constant and overturned theories the norm. As described in Tuesday’s post, it was only several years ago that results from the Kepler Space Telescope alerted scientists to the widespread presence of these super-Earths and sub-Neptunes, so the fluidity of the field is hardly surprising.

One well-respected researcher who is bullish on super-Earth biology is Harvard University astronomy professor Dimitar Sasselov. He argues that the logic of physics tells us that the “sweet spot” for planetary habitability is planets from the size of Earth to those perhaps as large as 1.4 Earth radii. Earth, he says, is actually small for a planet with life, and planets with a 1.2 Earth radii would probably be ideal.

I will return to his intriguing analysis, but first will catalog a bit of what scientists have detected or observed so far about super-Earths and sub-Neptunes. As a reminder, here’s the chart of Kepler exoplanet candidate and confirmed planets that orbit G, K and M main sequence stars put together by Mission Scientist for the Kepler Space Telescope Kepler Natalie Batalha.

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Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars, by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)

Expanding a bit:

  • While very large exoplanets were the first to be found because of the kind of instruments and techniques used in the search, the consensus view now is that small planets are much more plentiful. Future instruments will doubtless reveal a vast collection of exoplanets in the Earth-radius ballpark and smaller, but super-Earths and sub-Neptunes may well still dominate.
  • There’s a growing body of evidence that when planets are larger than roughly 1.5 Earth radii, they will likely be surrounded by a hyrogen and helium envelope dating back to the formation of the planet. The pull of the larger planets keeps the gases intact and often will render the planet essentially inert.
  • Roughly 70 percent of the main sequence stars in the galaxy are in the M dwarf category, smaller and less powerful stars that are known to have many exoplanets. Often they orbit close in to their sun and are packed together in a tight habitable zone. But many M dwarf planets in their habitable zones are like our moon – tidally locked so one side always faces the sun. Whether or not that rules them out in terms of habitability is now a hotly debated topic.

It’s quite amazing what has been learned about super-Earths, but compared to our knowledge of our solar system planets, we know very, very little. And what, after enormous effort, imagination and cost we do know? Generally speaking, gross measurements of mass, size, orbit period and density. They can tell scientists a lot about super-Earths and larger exoplanets, such as whether they are rocky, gaseous, and the mixtures in between. But characterizing them – and ultimately determining if they are at all capable of supporting life — really requires at a minimum that ability to measure what elements and compounds are in its atmosphere.

There are techniques for doing this: When an exoplanet passes in front of its star, the chemical make-up of the planet’s atmosphere can be analyzed by looking at how light either passes through or is absorbed by molecules, providing a telltale spectral reading of its contents.

trans_spec
Probing potassium in the atmosphere of HD 80606b with tunable filter transit spectrophotometry from the Gran Telescopio Canarias (European Space Observatory.)

Caroline Morley, of Jonathan Fortney’s group at the University of California at Santa Cruz, is one of those working to understand those measurements. But so far, the larger super-Earth and sub-Neptune planets are not cooperating.

“No spectral features are coming through, and so we’re limited in our characterizing,” Morley said. “This is a fundamental problem.” The spectral blanks, she and others are convinced, are the result of either thick clouds (probably made up of salts like zinc sulfide and potassium chloride) or a sooty hydrocarbon smog surrounding the planets. They keep the necessary stellar light from passing through in a way that would allow the presence of an enriched atmosphere, if present, to be identified and analyzed.

Like Morley, Robert Charnay and Victoria Meadows of the Virtual Planetary Laboratory at the University of Washington have been working to understand the opaque nature of the sub-Neptune planet Gliese 1214b in particular. They used a 3D circulation model to determine that salt clouds, which would form at lower altitudes, could nonetheless rise to the upper atmosphere and block any spectral readings.

These obstacles to analyzing the atmospheres of super-Earths were an initial surprise and have been a major frustration in the field. The James Webb Space Telescope may be able to peer through the clouds via detection of thermal emissions, so the 2018 arrival of the successor to the Hubble is eagerly awaited.

Caroline Morey of the University of California, Santa Cruz
Caroline Morey of the University of California, Santa Cruz (Jennifer Burt)

Based on what astronomers, planetary scientists, astrophysicists and others have been able to learn so far about the super-Earths to sub-Neptunes, the picture for potential habitability does not appear particularly bright. But there are other ways to assess that informal class of planets and come up with very different conclusions.

Dimitar Sasselov looks at astronomical problems from a more theoretical perspective, though he was a co-investigator for the Kepler telescope too. As Sasselov sees it, the basic physics of super-Earths, especially the smaller ones, actually favor life. The reason why is that it favors stability.

“There is no particular reason why a bigger planet might not be habitable,” he said. “When planets go bigger they get more stable, though certainly other problems will can and will arise. But when looking at some of the super-Earths, I contend they are as good for life as Earth, if not better.”

The additional stability comes in various forms. First is a less variable slant to the spin axis – its obliquity. Planets can get into trouble when that slant is highly changeable, as Sasselov says, pointing to Mars as an example. The planet’s steep and sometimes chaotic changes in the angle of its spin are believed to have caused dramatic climate changes.

Then there is the greater gravity that comes with a more massive planet. That increased gravity can have the effect of keeping an atmosphere from evaporating, a process that, among other things, exposes the planet to the charged particles coming from a parent star.

Dimitar Sallelov, professor of Astronomy at Harvard University and director of the Harvard Origins of Life Initiative
Dimitar Sasselov, professor of Astronomy at Harvard University and director of the Harvard Origins of Life Initiative (NASA)

And there is the increased likelihood of some kind of recycling of material from the planet surface back into the mantle, where it gets chemically enriched. Plate tectonics allows for that kind of chemical redistribution on Earth, but smaller planets are not known to have parallel processes.

With a more stable atmosphere and planetary slant, “the geochemistry of a planet has plenty of time to leap to biochemistry, and then to adapt,” he said. “Life is rooted in geochemistry and has to play by the rules of chemistry and physics.   There are many aspects to stability, and all benefit from a planet being Earth-sized or larger.”

As for Earth itself, he sees our planet as being uncomfortably close to that low edge of what makes a planet not very stable – not especially habitable.

There are, of course, limits to finding habitable conditions on most large planets. Sasselov agrees that planets bigger than 1.5 earth radii or so will tend to keep their primordial envelope of hydrogen and helium, which can have the effect of freezing everything on the planet in place and making life impossible. They will also tend to be fully gaseous rather than rocky.

But there will always be outliers, he said, and he has spent a lot of time working on one of the – the “Mega-Earth” Kepler 10c. It is a huge super-Earth with a surprisingly high ratio of rock, orbiting its sun in 45 days and breaking all the rules – such as they are now – about planet formation.

An artist concept shows the Kepler-10 system, home to two rocky planets. In the foreground is Kepler-10c, a planet that weighs 17 times as much as Earth and is more than twice as large in size. This discovery has planet formation theorists challenged to explain how such a world could have formed. (Harvard-Smithsonian Center for Astrophysics/David Aguilar)
An artist concept shows the Kepler-10 system, home to two rocky planets. In the foreground is Kepler-10c, a planet that weighs 17 times as much as Earth and is more than twice as large in size. This discovery has planet formation theorists challenged to explain how such a world could have formed. (Harvard-Smithsonian Center for Astrophysics/David Aguilar)

“We were very surprised to discover it,” said Sasselov, who was part of the Harvard-Smithsonian Center for Astrophysics team that first detected and analyzed it. “It was so large, so close to its sun, and much more massive than we would have predicted.”

It is a very hot planet and so not at all habitable, but it is a clear reminder that nature has ways of producing results that don’t appear at all plausible.

Sasselov said those anomalies may have to do with planet migration – that a large and gaseous Kepler 10c moved from the outer solar system into the close environs of its sun, and then lost some or all of its gas envelope from the heat and other solar activity. But the however it evolved – and since the system is more than 10 billion years old, it had a lot of time to evolve – it ended up a planet with 2.2 Earth radii with no apparent gas barriers around it and close to its sun. That, says Sasselov, makes it perfect to study with spectroscopy.

When NASA announced the huge mass of the planet in 2014, Kepler mission Batalha said: “Just when you think you’ve got it all figured out, nature gives you a huge surprise—in this case, literally.”

 

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Counting Our Countless Worlds

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The Milky Way has several hundred billion stars, and many scientists are now convinced it has even more planets and moons. (NASA)
The Milky Way is home to several hundred billion stars, and many scientists are now convinced it has even more planets and moons. (NASA)

Imagine counting all the people who have ever lived on Earth, well over 100 billion of them.

Then imagine counting all the planets now orbiting stars in our Milky Way galaxy , and in particular the ones that are roughly speaking Earth-sized. Not so big that the planet turns into a gas giant, and not so small that it has trouble holding onto an atmosphere.

In the wake of the explosion of discoveries about distant planets and their suns in the last two decades, we can fairly conclude that one number is substantially larger than the other.

Yes, there are many, many billions more planets in our one galaxy than people who have set foot on Earth in all human history. And yes, there are expected to be more planets in distant habitable zones as there are people alive today, a number upwards of 7 billion.

This is for sure a comparison of apples and oranges. But it not only gives a sense of just how commonplace planets are in our galaxy (and no doubt beyond), but also that the population of potentially habitable planets is enormous, too.   “Many Worlds,” indeed.

The populations of exoplanets identified so far, plotted according to the radius of the planet and how many days it takes to orbit. The circles in yellow represent planets found by Kepler, light blue by using ground-based radial velocity, and pink for transiting planets not found by Kepler, and green, purple and red other ground-based methods. (NASA Ames Research Center)
The populations of exoplanets identified so far, plotted according to the radius of the planet and how many days it takes to orbit. The circles in yellow represent planets found by Kepler, light blue by using ground-based radial velocity, and pink for transiting planets not found by Kepler, and green, purple and red other ground-based methods. (NASA Ames Research Center)

It was Ruslan Belikov, an astrophysicist at NASA’s Ames Research Center in Silicon Valley who provided this sense of scale.  The numbers are of great importance to him because he (and others) will be making recommendations about future NASA exoplanet-finding and characterization missions based on the most precise population numbers that NASA and the exoplanet community can provide.

Natalie Batalha, Mission Scientist for the Kepler Space Telescope mission and the person responsible for assessing the planet population out there, sliced it another way. When I asked her if her team and others now expect each star to have a planet orbiting it, she replied: “At least one.”

Kepler-186f was the first rocky planet to be found within the habitable zone -- the region around the host star where the temperature is right for liquid water. This planet is also very close in size to Earth. (NASA Ames/SETI Institute/JPL-Caltech)
Kepler-186f was the first rocky planet to be found within the habitable zone — the region around the host star where the temperature is right for liquid water. This planet is also very close in size to Earth. (NASA Ames/SETI Institute/JPL-Caltech)

I caught up with Belikov, Batalha and several dozen others intimately involved in cataloguing the vast menagerie of exoplanets at a “Hack Event” earlier this month at Ames. The goal of the three-day gathering was to find ways to improve the already high level of reliability and completeness regarding planets identified by Kepler.

It also provided an opportunity to learn more about how, exactly, these scientists can be so confident about the very large numbers of exoplanets and habitable zone exoplanets they describe. After all, the total number of confirmed exoplanets is a bit under 2,000 – a majority found by Kepler but hundreds of others by pioneering astronomers using ground-based telescopes and very different techniques. Kepler has another 3,000 planet candidates that scientists are in the process of analyzing and most likely confirming, but still. Four thousand is minuscule compared with two hundred billion.

Not everyone completely agrees that we’re ready to estimate such large numbers of exoplanets—suggesting that we need more data before making such important estimates — but the community consensus is that their extrapolations from current data are solid and scientific. And here is why:

The Kepler telescope looks out at a very small portion of the sky with a limited number of stars – about 190,000 of them during its four year survey. And it identifies planets based on the tiny dimming of stars when an object (almost always a planet) crosses between the star and the telescope.

The Kepler telescope looked constantly for four years at almost 200,000 stars in the Cygnus constellation. (Carter Roberts)
The Kepler telescope looked constantly for four years at almost 200,000 stars in the Cygnus & Lyra constellations.  Its lens is always open, by design. (Carter Roberts)

By identifying those 4,000-plus confirmed and candidate planets over four years, Kepler infers the existence of many, many more. As Batalha explained, a transit of the planet is only observable when the orbit is aligned with the telescope, and the probability of that alignment is very small. Kepler scientists refer to this as a “bias” in their observations, and it is one that can be quantified. For example, the probability that an Earth-Sun twin will be aligned in a transiting geometry is just 0.5%. For every one that Kepler detects, there are 200 others that didn’t transit simply because of the orientation of their orbits.

Then there’s the question of faintness and reliability. Kepler is looking out at stars hundreds, sometimes thousands of light years away.  The more distant a star, the fainter it is and the more difficult it is to gather measurements of –and especially dips in — brightness. When it comes to potentially habitable, Earth-sized planets, Batalha said that only 10,000 to 15,000 of the stars observed are bright enough for planets to be detectable even if they do transit the disk of their host star.

Here’s why: Detecting an Earth-sized planet would be roughly equivalent to capturing the image of a gnat as it crosses a car headlight shining one mile away. For a Jupiter-size planet, the bug would grow to only the size of a large beetle.

Add this bias to the earlier one, and you can see how the numbers swell so quickly. And since Kepler’s mission has been to provide a survey of planets in one small region – and not a census – this kind of statistical extrapolation is precisely what the mission is supposed to do.

There are numerous other detecting challenges posed by the dynamics of exoplanets, stars and the great distances. But then there are also innumerable challenges associated with the workings of the 95 megapixel CCD array that is collecting light for Kepler.   “Sensitivity dropouts” caused by those cosmic rays, horizontal “rolling bands” on the CCDs caused by temperature changes in the electronics, “optical ghosts” from binary stars that create false signals of transits on nearby stars — they are some of the many instrument artifacts that can be mistaken as a drop in light coming from a planet. Kepler’s data processing pipeline, much of which has been transferred over to the NASA Ames supercomputer, has the job of sorting all this out.

 

After the CCDs on the Kepler telescope record the light from stars in its viewing field, the data is sent back to Earth and goes through numerous steps before possibly delivering a “Kepler object of interest,” and possibly a planet candidate. Pleiades is the Ames supercomputer. (NASA Ames)
After the CCDs on the Kepler telescope record the light from stars in its viewing field, the data is sent back to Earth and goes through numerous steps before possibly delivering a “Kepler object of interest,” and possibly a planet candidate. Pleiades is the Ames supercomputer. (NASA Ames)

Adding to the challenge, said Jon Jenkins, a Kepler co-investigator at Ames and the science lead for the pipeline development, is that the stars viewed by Kepler turned out to be themselves “noisier” than expected. Stars naturally vary in their overall brightness, and the data processing pipeline had to be upgraded to account for that changeability.  But that stellar noise has played a key role in keeping Kepler from seeing some of the small planet transits that the team hoped to detect.

What the Hack event and other parallel efforts are doing is finding ways to, as Jenkins put it, “dig into the noise…to move towards the hairy edge of what our data can show.” The final goal: “To come up with the newest, best washer we can to clean the data and come out with an improved catalog of sparkling planets.”

All the data that will come from the primary Kepler mission, which came to a halt in the summer of 2013, has been collected and analyzed already on a first round. But now the entire pipeline of data is going to be reprocessed with its many improvements so the researchers can dig deeper into data trove. Batalha said they hope to find planets – especially Earth-sized planets – this way.

One of the key techniques to measure the performance of Kepler’s analysis pipeline is to inject fake transit signals into the data and see if it picks up their presence. As Batalha explained, this provides another way to gauge the biases in the system, its efficiency at detecting the planets that it could and should see. “If we inject 100 fake things into the pipeline and find 90 of them, that’s means we’re 90 percent complete.” She said the number would then be worked into the calculations of how many planets are out there, and how many of certain sizes will be caught and missed.

Natalie Batalha is the Chief Scientist for the Kepler mission, while announcing the discovery of Kepler’s first rocky planet, Kepler-10b, in January 2011.

So the Hack Event, which brought together astrophysicists, planetary scientists and computer hakers, was designed to come up with ways to improve Kepler’s completeness (seeing everything there to be seen) and reliability (the likelihood that the signal comes from a planet and not an instrument artifact or non-planetary phenomena in space). By computing both the completeness and reliability, scientists are confident that they can eliminate the observation biases and transform the discovery catalog into a directory of actual planets.

This is one of the key accomplishments of the Kepler mission – making it scientifically possible to say that there are billions and billions of planets out there. What’s more, the increased power of Kepler allowed for the discovery of smaller planets, which are now known to make up the bulk of the exoplanets. And while the number of Earth-sized planets detected in that habitable zone is small – around thirty – that’s still quite a remarkable feat. And remember, Kepler is looking at but one small sliver of the sky.

The twelve exoplanets detected so far closest to Earth in size, lined up with the type of stars they orbit. (NASA Ames)
The twelve exoplanets detected and confirmed so far closest to Earth in size, lined up with the type of stars they orbit. (NASA Ames)

Why does it matter how many exoplanets are out there, how many are rocky and Earth-sized, and how many within habitable zones? The last twenty years of exoplanet hunting, after all, has made clear that there are an essentially infinite number of them in the universe, and untold billions in our galaxy.

The answer lies in the insatiable human desire to know more about the world writ large, and how and why different stars have very different solar systems. But more immediately, there’s the need to know how to best design and operate future planet-finding missions. If the goal is to learn how to characterize exoplanets – identify components of their atmospheres, learn about their weather, their surfaces and maybe their cores – then scientists and engineers need to know a lot more about where planets generally, and some specifically, can be found. And those planet demographics just might open some surprising possibilities.

For instance, Belikov and his Ames colleague Eduardo Bendek have proposed a NASA “small explorer” (under $175 million) mission to launch a 30-to-45 centimeter mirror designed to look for Earth-sized planets only at our nearest stellar neighbor, Alpha Centauri. That’s as small a telescope as you can buy off-the-shelf.

Alpha Centauri is the closest star system to our Solar System at about 4.37 lightyears away. (NASA/Hubble Space Telescope)
Alpha Centauri is the closest star system to our Solar System at about 4.37 lightyears away. (NASA/Hubble Space Telescope)

Alpha Centauri is a two-star system, and until recently researchers doubted that binaries like it would have orbiting planets. But Kepler and other planet hunters have found that planets are relatively common around binaries, making Alpha Centauri a better target than earlier imagined.

To make it a truly viable project, ACESat – the Alpha Centauri Exoplanet Satellite – requires something else: a scientifically sound estimate of the likelihood that any star in our galaxy would have an Earth-sized planet in its system. Estimates so far have ranged from 10 percent to 50 percent, but Belikov said newer data is encouraging.

“If that number becomes more firm and approaches 50 percent, then an Alpha Centauri-only mission makes a great deal of sense,” he said. “For a small investment, we could have a real possibility of detecting a planet very close by.”

Intriguing, and an insight into how new space missions are designed based on the science already completed. Both NASA and the European Space Agency have plans to launch three significant exoplanet missions within the decade, and the powerful James Webb Space Telescope will launch in 2018 with some known and undoubtedly some not yet understood capabilities for exoplanet discovery. And perhaps most important, NASA is about to study how a potential mission in the 2030s could be designed with the specific purpose of directly imaging exoplanets – the gold standard for the field. All are being designed based on current exoplanet understandings, including the abundance calculations enabled by the Kepler mission’s observations.

Almost 2,000 exoplanets have now been identified, more than half by Kepler. Another 3,000 exoplanet candidates await confirmation. (NASA Ames)
Almost 2,000 exoplanets have now been detected and confirmed, more than half by Kepler. Another 3,000 exoplanet candidates await confirmation. (NASA Ames)

Future posts will dig deeper into a fair number of the subjects raised here, but for now this much is clear: Our galaxy has many billions of planets, and the process of detecting them is robust and on-going, the process of characterizing them has begun, and all the signs point towards the presence of enormous numbers of planets in habitable zones that, in the biggest picture at least, could possibly support life.

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The Exoplanet Era

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Many, and perhaps most stars have solar systems with numerous planets, as in this artist rendering of Kepler 11. (NASA)

Throughout the history of science, moments periodically arrive when new fields of knowledge and discovery just explode.

Cosmology was a kind of dream world until Edwin Hubble established that the universe was expanding, and doing so at an ever-faster rate. A far more vibrant and scientific discipline was born. On a more practical level, it was only three decades ago that rudimentary personal computers were still a novelty, and now computer-controlled, self-driving cars are just on the horizon. And not that long ago, genomics and the mapping of the human genome also went into hyperspeed, and turned the mysterious into the well known.

Most frequently, these bursts of scientific energy and progress are the result of technological innovation, coupled with the far-seeing (and often lonely and initially unsupported) labor and insights of men and women who are simply ahead of the curve.

We are at another of those scientific moments right now, and the subject is exoplanets – the billions (or is it billions of billions?) of planets orbiting stars other than our sun.

The 20th anniversary of the breakthrough discovery of the first exoplanet orbiting a sun, 51 Pegasi B, is being celebrated this month with appropriate fanfare. But while exoplanet discovery remains active and planet hunters increasingly skilled and inventive, it is no longer the edgiest frontier.

Now, astronomers, astrophysicists, astrobiologists, planetary scientists, climatologists, heliophysicists and many more are streaming into a field made so enticing, so seemingly fertile by that discovery of the apparent ubiquitiousness of exoplanets.

The new goal: Identifying the most compelling mysteries of some of those distant planets, and gradually but inexorably finding ever-more inventive ways to solve them. This is a thrilling task on its own, but the potential prize makes it into quite an historic quest. Because that prize is the identification of extraterrestrial life.

The presence of life beyond Earth is something that humans have dreamed about forever – with a seemingly intuitive sense that there just had to be other planets out there, and that it made equal sense that some of them supported life. Hollywood was on to this long ago, but now we have the beginning technology and fast-growing knowledge to transform that intuitive sense of life out there into a working science.

The thin gauzy rim of the planet in foreground is an illustration of its atmosphere. (NASA’s Goddard Space Flight Center)
The thin gauzy rim of the planet in foreground is an illustration of its atmosphere. (NASA’s Goddard Space Flight Center)

Already the masses and orbits of several thousand exoplanets have been measured. Some planets have been identified as rocky like Earth (as opposed to gaseous like Jupiter.) Some have been found in what the field calls “habitable zones” – regions around distant suns where liquid water could plausibly run on a surface –as it does on Earth and once did on Mars. And some exoplanets have even been determined to have specific compounds – carbon dioxide, water, methane, even oxygen – in their atmospheres.

This and more is what I will be exploring, describing, hopefully bringing to life through an on-going examination of this emerging field of science and the inventive scientists working to understand planets and solar systems many light-years away. Theirs is a daunting task for sure, and progress may be halting. But many scientists are convinced that the goal is entirely within reach – that based on discoveries already made, the essential dynamics and characteristics of very different kinds of planets and solar systems are knowable. Thus the name of this offering: “Many Worlds.”

Artist rendering of early stages of planet formation in the swirl and debris of the disk of a new star. (NASA/JPL-Caltech)
Artist rendering of early stages of planet formation in the swirl and debris of the disk of a new star. (NASA/JPL-Caltech)

I was first introduced to, and captivated by, this cosmic search in a class for space journalists taught by scientists including Sara Seager, a dynamic young professor of physics and planetary science at M.I.T., a subsequently-selected MacArthur “genius,” and a pioneer in the field not of discovering exoplanets, but of characterizing them and their atmospheres. And based on her theorizing and the observations of many others, she was convinced that this characterizing would lead to the discovery of very distant extraterrestrtial life, or at least to the discovery of planetary signatures that make the presence of life highly probable. Just this week, she predicted the discovery could take place within a decade.

It was in 2010 that she began her book “Exoplanet Atmospheres” with the statement: “A new era in planetary science is upon us.” I would take it further: A new era has arrived in the human drive to understand the universe and our place in it. Exoplanets and their solar systems are a magnet to young scientists, says Paul Hertz, the head of NASA’s Astrophysics Division. Almost a third of the papers presented at astronomy conferences these days involve exoplanets, he said, and “it’s hard to find scientists in our field under thirty not working on exoplanets.” Go to a major geology conference, or a planetary science meeting, and much the same will be true.

And why not? I think of this moment as akin to the time in the 17th century when early microscopes revealed a universe of life never before seen. So many new questions to ask, so many discoveries to make, so much exciting and ultimately world-changing science ahead.

But the challenge of characterizing exoplanets and some day identifying signs of life does not lend itself to the kind of solitary or small group work that characterized microbiology (think the breakthrough NASA Kepler mission and the large team needed to make it reality and to analyze its results.) Not only does it require costly observatories and telescopes and spectrometers, but it also needs the expertise that scientists from different fields can bring to the task – rather like the effort to map the human genome.

That is the organizing logic of astrobiology – the more general hunt for life elsewhere in our solar system and far beyond, alongside the search for clues into how life may have started on our planet. NASA is eager to encourage that same spirit in the more specific but nonetheless equally sprawling exploration of exoplanets, their atmospheres, their physical makeup, their climates, their suns, their neighborhoods.

The Earth alongside “Super-Earth-” sized exoplanets identified with the Kepler Space Telescope. (NASA Ames / JPL-Caltech)
The Earth alongside “Super-Earth-” sized exoplanets identified with the Kepler Space Telescope. (NASA Ames / JPL-Caltech)

The result was the creation this summer of the the Nexus for Exoplanet System Science (NExSS), a group that will be led by 17 teams of scientists from around the country already working on some aspect of the rich exoplanet opportunity. The group was selected from teams that had applied for grants from NASA’s Astrobiology Institute, an arm of its larger NASA Astrobiology Program, as well as other NASA programs in the Planetary Sciences, Astrophysics and Astronomy divisions.

Their mandate is to spark new approaches in the effort to understand exoplanets by identifying areas without consensus in the broader community, and then fostering collaborations here and abroad to address those issues. “Many Worlds” grew out of the NExSS initiative, and will chronicle and explain the efforts of some team members as they explore how exo-plants and exo-creatures might be detected; what can be learned from afar about the surfaces and cores of exoplanets and how both play into the possibility of faraway life; the presence and dynamics of exo-weather, what we can learn about exoplanets from our own planet and solar system, and so much more.

A few of the teams are small, but many are quite large, established and mature – perhaps most especially the Virtual Planetary Laboratory at the University of Washington, and run by Victoria Meadows. Since 2001, the virtual lab has collaborated with researchers representing many disciplines, and from as many as 20 institutions, to understand what factors might best predict whether an exoplanet harbors life, using Earth as a model.

But just as I will be venturing beyond NExSS in my writing about this new era of exploration, so too will NExSS be open to the involvement of other scientists in the field. The original group has been tasked with identifying an agenda of sorts for NASA exoplanet missions and efforts ahead. But its aim is to be inclusive and its conclusions and recommendations will only be as useful and important as the exoplanet community writ large determines them to be.

The Carina Nebula, one of many regions where stars come together and planets later form made out of the surrounding dust, gas and later rock. (NASA, ESA, and the Hubble SM4 ERO Team)
The Carina Nebula, one of many regions where stars come together and planets later form made out of the surrounding dust, gas and later rock. (NASA, ESA, and the Hubble SM4 ERO Team)

This is a moment pregnant with promise. Systematically investigating exoplanets and their environs is an engine for discovery and a pathway into that largest question of whether or not we are alone in the universe.

Will scientists some day find worlds where donkeys talk and pigs can fly (as at least one “everything is possible” philosopher has posited)? Unlikely.

But just as microscopes and the scientists using them led to the science of microbiology and most of modern medicine, so too are our orbiting observatories, Earth-based telescopes and the scientists who analyze their results are regularly opening up a world of myriad and often surprising marvels.

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