In the biggest haul ever of new exoplanets, scientists with NASA’s Kepler mission announced the confirmation of 1,284 additional planets outside our solar system — including nine that are relatively small and within the habitable zones of their host stars. That almost doubles the number of these treasured rocky planets that orbit their stars at distances that could potentially support liquid water and potentially life.
Prior to today’s announcement, scientists using Kepler and all other exoplanet detection approaches had confirmed some 2,100 planets in 1,300 planetary systems. So this is a major addition to the exoplanets known to exist and that are now available for further study by scientists.
These detections comes via the Kepler Space Telescope, which collected data on tiny decreases in the output of light from distant stars during its observing period between 2009 and 2013. Those dips in light were determined by the Kepler team to be planets crossing in front of the stars rather than impostors to a 99 percent-plus probability.
As Ellen Stofan, chief scientist at NASA Headquarters put it, “This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth.”
The primary goals of the Kepler mission are to determine the demographics of exoplanets in the galaxy, and more specifically to determine the population of small, rocky planets (less than 1.6 times the size of Earth) in the habitable zones of their stars. While orbiting in such a zone by no means assures that life is, or was, ever present, it is considered to be one of the most important criteria.
The final Kepler accounting of how likely it is for a star to host such an exoplanet in its habitable zone won’t come out until next year. But by all estimations, Kepler has already jump-started the process and given a pretty clear sense of just how ubiquitous exoplanets, and even potentially habitable exoplanets, appear to be.
“They say not to count our chickens before they’re hatched, but that’s exactly what these results allow us to do based on probabilities that each egg (candidate) will hatch into a chick (bona fide planet),” said Natalie Batalha, co-author of the paper in the Astrophysical Journal and the Kepler mission scientist at NASA’s Ames Research Center.
“This work will help Kepler reach its full potential by yielding a deeper understanding of the number of stars that harbor potentially habitable, Earth-size planets — a number that’s needed to design future missions to search for habitable environments and living worlds.”
Batalha said that based on observations and statistics the Kepler mission has produced so far, we can expect that there are some 10 billion relatively small, rocky (and potentially habitable) planets in our galaxy. And with those numbers in mind, she said, the closest is likely to be in the range of 11 light years away.
She said that all of the exoplanets found in habitable zones are in the “exoplanet Hall of Fame.” But she said two of the newly announced planets in habitable zones, Kepler 1286b and Kepler 1628b, joined two previous exoplanets of particular interest either because of their size (close to that of Earth) or their Earth-like distance from suns rather like ours.
Batalha said a new and finely-tuned software pipeline has been developed to better analyze the data collected during those four years of Kepler observations. Asked if the final Kepler catalogue of exoplanets, expected to be finished next summer, would increase the current totals of exoplanets found, she replied: “It wouldn’t surprise me if we had hundreds more to add.”
Once the Kepler exoplanet list is updated, scientists around the world will begin to study some of the most surprising, enticing, and significant finds. Kepler can tell scientists only the location of a planet, its mass and its distance from the host star. So the job of further characterizing the planets — and ultimately determining if any are indeed potentially habitable — requires other telescopes and techniques.
Nonetheless, Kepler’s ability to give scientists a broad picture of the distribution of exoplanets — to find large numbers of them rather than, as pre-Kepler, one or two at a time — has been revolutionary. It has also been remarkably speedy, thanks in large part to an automated system of analyzing transit data devised by Tim Morton, a research scientist at Princeton University,
“Planet candidates can be thought of like bread crumbs,” Morton said in a NASA teleconference. “If you drop a few large crumbs on the floor, you can pick them up one by one. But, if you spill a whole bag of tiny crumbs, you’re going to need a broom. This statistical analysis is our broom.”
Kepler identified another 1,327 candidates that are very likely to be exoplanets, but didn’t meet the 99 percent certainty level required to be deemed an exoplanet.
A large percentage of the newly confirmed planets are either “super-Earths” or “sub-Neptunes” — planets in a size range absent in our solar system. Initially, the widespread presence of exoplanets of these dimensions was a puzzle to the exoplanet community, but now the puzzle is more why they are absent in our system.
Despite the abundance of these exoplanets — which are believed to be mostly gas or ice giants — scientists are convinced there are considerably more rocky, even Earth-sized planets that current telescopes can’t detect.
The primary Kepler mission focused on one small piece of the sky — about 0.25 percent of it — and a distant part at that. It watched nonstop for transiting planets in that space for four years, watching unblinkingly at some 150.000 stars. The result has been a treasure trove of data that can then be broadened statistically to tell us about the entire galaxy.
So Kepler has revolutionized our understanding of the galaxy and what’s in it, and has proven once and for all that exoplanets are common. But the individual planets that it has detected are unlikely to be the ones that allow for breakthroughs in terms of sniffing out what chemicals are in their atmospheres — an essential process for determining if a potentially habitable planet actually has some of the ingredients for life.
This is because Kepler was looking far into the cosmos, between 600 and 3,000 light years from our sun. While the telescope identified almost 5,000 “candidate planets” during its four years of primary operation — and now more than 2,200 confirmed planets — the planets are generally considered too distant for the more precise follow-up observing needed to understand their atmospheres and chemical make-ups.
This work will fall to ground-based telescopes looking at nearer stars, and to future generations of American and European space telescopes using the transit method of detection pioneered by Kepler. (See graphic above.) The next space satellite in line is NASA’s Transiting Exoplanet Survey Satellite (TESS), which is scheduled to launch in 2017 and will focus on planets orbiting much closer and brighter stars. The long-awaited James Webb Space Telescope, due to launch in 2018, also has the potential to study exoplanets with a precision, and in wavelengths, not available before.
NASA has begun development of the more sophisticated Wide Field Infrared Survey Satellite (WFIRST) to further exoplanet research in the 2020s, and has set up formal science and technology definition teams to plan for a possible flagship exoplanet mission for the 2030s. That mission would potentially have the power and techniques to determine whether an exoplanet actually has the components, or the presence, of life.
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!”
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.)
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.
But whatever happens with K2, TRAPPIST-1 is now an astronomical “star” and will no doubt be getting scientific attention of all kinds.
An earlier version of this article was accidently published last week before it was completed. This is the finished version, with information from this week’s AAS annual conference.
Let’s face it: the field of exoplanets has a significant deficit when it comes to producing drop-dead beautiful pictures.
We all know why. Exoplanets are just too small to directly image, other than as a miniscule fraction of a pixel, or perhaps some day as a full pixel. That leaves it up to artists, modelers and the travel poster-makers of the Jet Propulsion Lab to help the public to visualize what exoplanets might be like. Given the dramatic successes of the Hubble Space Telescope in imaging distant galaxies, and of telescopes like those on the Cassini mission to Saturn and the Mars Reconnaissance Orbiter, this is no small competitive disadvantage. And this explains why the first picture of this column has nothing to do with exoplanets (though billions of them are no doubt hidden in the image somewhere.)
The problem is all too apparent in these two images of Pluto — one taken by the Hubble and the other by New Horizons telescope as the satellite zipped by.
Pluto is about 4.7 billion miles away. The nearest star, and as a result the nearest possible planet, is 25 trillion miles away. Putting aside for a minute the very difficult problem of blocking out the overwhelming luminosity of a star being cross by the orbiting planet you want to image, you still have an enormous challenge in terms of resolving an image from that far away.
While current detection methods have been successful in confirming more than 2,000 exoplanets in the past 20 years (with another 2,000-plus candidates awaiting confirmation or rejection), they have been extremely limited in terms of actually producing images of those planetary fireflies in very distant headlights. And absent direct images — or more precisely, light from those planets — the amount of information gleaned about the chemical makeup of their atmospheres as been limited, too.
But despite the enormous difficulties, astronomers and astrophysicist are making some progress in their quest to do what was considered impossible not that long ago, and directly image exoplanets.
In fact, that direct imaging — capturing light coming directly from the sources — is pretty uniformly embraced as the essential key to understanding the compositions and dynamics of exoplanets. That direct light may not produce a picture of even a very fuzzy exoplanet for a very long time to come, but it will definitely provide spectra that scientists can read to learn what molecules are present in the atmospheres, what might be on the surfaces and as a result if there might be signs of life.
There has been lots of technical and scientific debate about how to capture that light, as well as debate about how to convince Congress and NASA to fund the search. What’s more, the exoplanet community has a history of fractious internal debate and competition that has at times undermined its goals and efforts, and that has been another hotly discussed subject. (The image of a circular firing squad used to be a pretty common one for the community.)
But a seemingly much more orderly strategy has been developed in recently years and was on display at the just-completed American Astronomical Society annual meeting in Florida. The most significant breaking news was probably that NASA has gotten additional funds to support a major exoplanet direct imaging mission in the 2020s, the Wide Field Infrared Survey Telescope (WFIRST), and that the agency is moving ahead with a competition between four even bigger exoplanet or astrophysical missions for the 2030s. The director of NASA Astrophysics, Paul Hertz, made the formal announcements during the conference, when he called for the formation of four Science and Technology Definition Teams to assess in great detail the potentials and plausibilities of the four possibilities.
Putting it into a broader perspective, astronomer Natalie Batalha, science lead for the Kepler Space Telescope, told a conference session on next-generation direct imaging that “with modern technology, we don’t have the capability to image a solar system analog.” But, she said, “that’s where we want to go.”
And the road to discovering exoplanets that might actually sustain life may well require a space-based telescope in the range of eight to twelve meters in radius, she and others are convinced. Considering that a very big challenge faced by the engineers of the James Webb Space Telescope (scheduled to launch in 2018) was how to send a 6.5 meter-wide mirror into space, the challenges (and the costs) for a substantially larger space telescope will be enormous.
We will come back in following post to some of these plans for exoplanet missions in the decades ahead, but first let’s look at a sample of the related work done in recent years and what might become possible before the 2020s. And since direct imaging is all about “seeing” a planet — rather than inferring its existence through dips in starlight when an exoplanet transits, or the wobble of a sun caused by the presence of an orbiting ball of rock (or gas) — showing some of the images produced so far seems appropriate. They may not be breath-taking aesthetically, but they are remarkable.
There is some debate and controversy over which planets were the first to be directly imaged. But all agree that a major breakthrough came in 2008 with the imaging of the HR8799 system via ground-based observations.
First, three Jupiter-plus gas giants were identified using the powerful Keck and Gemini North infrared telescopes on Mauna Kea in Hawaii by a team led by Christian Marois of the National Research Council of Canada’s Herzberg Institute of Astrophysics. That detection was followed several years later the discovery of a fourth planet and then by the release of the surprising image above, produced with the quite small (4.9 foot) Hale telescope at the Palomar Observatory outside of San Diego.
As is the case for all planets directly imaged, the “pictures” were not taken as we would with our own cameras, but rather represent images produced with information that is crunched in a variety of necessary technical ways before their release. Nonetheless, they are images in a way similar the iconic Hubble images that also need a number of translating steps to come alive.
Because light from the host star has to be blocked out for direct imaging to work, the technique now identifies only planets with very long orbits. In the case of HR8799, the planets orbit respectively at roughly 24, 38 and 68 times the distance between our Earth and sun. Jupiter orbits at about 5 times the Earth-sun distance.
In the same month as the HR8799 announcement, another milestone was made public with the detection of a planet orbiting the star Formalhaut. That, too, was done via direct imagining, this time with the Hubble Space Telescope.
Signs of the planet were first detected in 2004 and 2006 by a group headed by Paul Kalas at the University of California, Berkeley, and they made the announcement in 2008. The discovery was confirmed several years later and tantalizing planetary dynamics began to emerge from the images (all in false color.) For instance, the planet appears to be on a path to cross a vast belt of debris around the star roughly 20 years from now. If the planet’s orbit lies in the same plane with the belt, icy and rocky debris could crash into the planet’s atmosphere and cause interesting damage.
The region around Fomalhaut’s location is black because astronomers used a coronagraph to block out the star’s bright glare so that the dim planet could be seen. This is essential since Fomalhaut b is 1 billion times fainter than its star. The radial streaks are scattered starlight. Like all the planets detected so far using some form of direct imaging, Fomalhaut b if far from its host star and completes an orbit every 872 years.
Adaptive optics of the Gemini Planet Imager, at the Gemini South Observatory in Chile, has been successful in imaging exoplanets as well. The GPI grew out of a proposal by the Center for Adaptive Optics, now run by the University of California system, to inspire and see developed innovative optical technology. Some of the same breakthroughs now used in treating human eyes found their place in exoplanet astronomy.
The Imager, which began operation in 2014, was specifically created to discern and evaluate dim, newer planets orbiting bright stars using a different kind of direct imaging. It is adept at detecting young planets, for instance, because they still retain heat from their formation, remain luminous and visible. Using the GPI to study the area around the y0ung (20-million-year-old) star 51 Eridiani, the team made their first exoplanet discovery in 2014.
By studying its thermal emissions, the team gained insights into the planet’s atmospheric composition and found that — much like Jupiter’s — it is dominated by methane. To date, methane signatures have been weak or absent in directly imaged exoplanets.
James Graham, an astronomer at the University of California, Berkeley, is the project leader for a three-year GPI survey of 600 stars to find young gas giant planets, Jupiter-size and above.
“The key motivation for the experiment is that if you can detect heat from the planet, if you can directly image it, then by using basic science you can learn about formation processes for these planets.” So by imaging the planets using these very sophisticated optical advances, scientists go well beyond detecting exoplanets to potentially unraveling deep mysteries (even if we still won’t know what the planets “look like” from an image-of-the-day perspective.
The GPI has also detected a second exoplanet, shown here at different stages of its orbit:
A next big step in direct imaging of exoplanets will come with the launch of the James Webb Space Telescope in 2018. While not initially designed to study exoplanets — in fact, exoplanets were just first getting discovered when the telescope was under early development — it does now include a coronagraph which will substantially increase its usefulness in imaging exoplanets.
As explained by Joel Green, a project scientist for the Webb at the Space Telescope Science Institute in Baltimore, the new observatory will be able to capture light — in the form of infrared radiation– that will be coming from more distant and much colder environments than what Hubble can probe.
“It’s sensitive to dimmer things, smaller planets that are more earth-sized. And because it can see fainter objects, it will be more help in understanding the demographics of exoplanets. It uses the infrared region of the spectrum, and so it can look better into the cloud levels of the planets than any telescope so far and see deeper.”
These capabilities and more are going to be a boon to exoplanet researchers and will no doubt advance the direct imaging effort and potentially change basic understandings about exoplanets. But it is not expected produce gorgeous or bizarre exoplanet pictures for the public, as Hubble did for galaxies and nebulae. Indeed, unlike the Hubble — which sees primarily in visible light — Webb sees in what Green said is, in effect, night vision. And so researchers are still working on how they will produce credible images using the information from Webb’s infrared cameras and translating them via a color scheme into pictures for scientists and the public.
Another compelling exoplanet-imaging technology under study by NASA is the starshade, or external occulter, a metal disk in the shape of a sunflower that might some day be used to block out light from host stars in order to get a look at faraway orbiting planets. MIT’s Sara Seager led a NASA study team that reported back on the starshade last year in a report that concluded it was technologically possible to build and launch, and would be scientifically most useful. If approved, the starshade — potentially 100 feet across — could be used with the WFIRST telescope in the 2020s. The two components would fly far separately, as much as 35,000 miles away from each other, and together could produce breakthrough exoplanet direct images.
Here is a link to an animation of the starshade being deployed: http://planetquest.jpl.nasa.gov/video/15
The answer, then, to the question posed in the title to this post — “How Will We Know What Exoplanets Look Like, and When?”– is complex, evolving and involves a science-based definition of what “looking like” means. It would be wonderful to have images of exoplanets that show cloud formations, dust and maybe some surface features, but “direct imaging” is really about something different. It’s about getting light from exoplanets that can tell scientists about the make-up of those exoplanets and their atmospheres, and ultimately that’s a lot more significant than any stunning or eerie picture.
And with that difference between beauty and science in mind, this last image is one of the more striking ones I’ve seen in some time.
It was taken at the Las Campanas Observatory in Chile last year, during a night of stargazing. Although the observatory is in the Atacama Desert, enough moisture was present in the atmosphere to create this lovely moon-glow.
But working in the observatory that night was Carnegie’s pioneer planet hunter Paul Butler, who uses the radial velocity method to detect exoplanets. But to do that he needs to capture light from those distant systems. So the night — despite the beautiful moon-glow — was scientifically useless.
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 paper 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 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 Astronomy & Astrophysics 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.
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.”
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.”
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.
(Find a panel discussion about the brown dwarf-exoplanet connection here.)
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.
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.
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.
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.
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.
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.
“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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