A New Frontier for Exoplanet Hunting

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The spectrum from the newly-assembled EXtreme PREcision Spectrometer (EXPRES)  shines on Yale astronomy professor Debra Fischer, who is principal investigator of the project. The stated goal of EXPRES is to find many Earth-size planets via the radial velocity method — something that has never been done. (Ryan Blackman/Yale)

The first exoplanets were all found using the radial velocity method of measuring the “wobble” of a star — movement caused by the gravitational pull of an orbiting planet.

Radial velocity has been great for detecting large exoplanets relatively close to our solar system, for assessing their mass and for finding out how long it takes for the planet to orbit its host star.

But so far the technique has not been able to identify and confirm many Earth-sized planets, a primary goal of much planet hunting.  The wobble caused by the presence of a planet that size has been too faint to be detected by current radial velocity instruments and techniques.

However, a new generation of instruments is coming on line with the goal of bringing the radial velocity technique into the small planet search.  To do that, the new instruments, together with their telescopes. must be able to detect a sun wobble of 10 to 20 centimeters per second.  That’s quite an improvement on the current detection limit of about one meter per second.

At least three of these ultra high precision spectrographs (or sometimes called spectrometers) are now being developed or deployed.  The European Southern Observatory’s ESPRESSO instrument has begun work in Chile; Pennsylvania State University’s NEID spectrograph (with NASA funding) is in development for installation at the Kitt Peak National Observatory in Arizona; and the just-deployed EXPRES spectrograph put together by a team led by Yale University astronomers (with National Science Foundation funding) is in place at the Lowell Observatory outside of Flagstaff, Arizona.

The principal investigator of EXPRES, Debra Fischer, attended the recent University of Cambridge Exoplanets2 conference with some of her team, and there I had the opportunity to talk with them. We discussed the decade-long history of the instrument, how and why Fischer thinks it can break that 1-meter-per-second barrier, and what it took to get it into attached and working.

 

This animation shows how astronomers use very precise spectrographs to find exoplanets. As the planet orbits its gravitational pull causes the parent star to move back and forth. This tiny radial motion shifts the observed spectrum of the star by a correspondingly small amount because of the Doppler shift. With super-sensitive spectrographs the shifts can be measured and used to infer details of a planet’s mass and orbit. ESO/L. Calçada)

One of the earliest and most difficult obstacles to the development of EXPRES, Fischer told me, was that many in the astronomy community did not believe it could work.

Their view is that precision below that one meter per second of host star movement cannot be measured accurately.  Stars have flares, sunspots and a generally constant churning, and many argue that the turbulent nature of stars creates too much “noise” for a precise measurement below that one-meter-per-second level.

Yet European scientists were moving ahead with their ESPRESSO ultra high precision instrument aiming for that 10-centimeter-per-second mark, and they had a proven record of accomplishing what they set out to do with spectrographs.

In addition to the definite competiti0n going on, Fisher also felt that radial velocity astronomers needed to make that leap to measuring small planets “to stay in the game” over the long haul.

She arrived at Yale in 2009 and led an effort to build a spectrograph so stable and precise that it could find an Earth-like planet.  To make clear that goal, the instrument is at the center of a project called “100 Earths.”

Building on experience gained from developing two earlier spectrographs, Fischer and colleagues began the difficult and complicated process of getting backers for EXPRES, of finding a telescope observatory that would house it (The Discovery Channel Telescope at Lowell) and in the end adapting the instrument to the telescope.

And now comes the actual hard part:  finding those Earth-like planets.

As Fischer described it:  “We know from {the Kepler Telescope mission} that most stars have small rocky planets orbiting them.  But Kepler looked at stars very far away, and we’ll be looking at stars much, much closer to us.”

Nonetheless, those small planets will still be extremely difficult to detect due to all that activity on the host suns.

 

EXPRES in its vacuum-sealed chamber at the Lowell Observatory. will help detect Earth-sized planets in neighboring solar systems. (Ryan Blackman/Yale)

 

 

The 4.3 meter Discovery Channel Telescope in the Lowell Observatory in Arizona.  The photons collected by the telescope are delivered via optical fiber to the EXPRES instrument. (Boston University)

Spectrographs such as EXPRES are instruments astronomers use to study light emitted by planets, stars, and galaxies.

They are connected to either a ground-based or orbital telescope and they stretch out or split a beam of light into a spectrum of frequencies.  That spectrum is then analyzed to determine an object’s speed, direction, chemical composition, or mass.  With planets, the work involves determining (via the Doppler shift seen in the spectrum) whether and how much a sun is moving to and away from Earth due to the pull of a planet.

As Fisher and EXPRES postdoctoral fellow John Brewer explained it, the signal (noise) coming from the turbulence of the star is detectably different from the signal made by the wobble of a star due to the presence of an orbiting planet.

While these differences — imprinted in the spectrum captured by the spectrograph — have been known for some time, current spectrographs haven’t had sufficient resolving power to actually detect the difference.

If all works as planned for the EXPRES, Espresso and NEID spectrographs, they will have that necessary resolving power and so can, in effect, filter out the noise from the sun and identify what can only come from a planet-caused wobble.  If they succeed, they provide a major new pathway to  for astronomers to search for Earth-sized worlds.

“This is my dream machine, the one I always wanted to build,” Fischer said. “I had a belief that if we went to higher resolution, we could disentangle (the stellar noise from the planet-caused wobble.)

“I could still be wrong, but I definitely think that trying was the right choice to make.”

This image shows spectral data from the first light last December of the ESPRESSO instrument on ESO’s Very Large Telescope in Chile. The light from a star has been dispersed into its component colors. This view has been colorised to indicate how the wavelengths change across the image, but these are not exactly the colors that would be seen visually. (ESO/ESPRESSO)

While Fischer and others have very high hopes for EXPRES, it is not the sort of  big ticket project that is common in astronomy.  Instead, it was developed and built primarily with a $6 million grant from National Science Foundation.

It was completed on schedule by the Yale team, though the actual delivering of EXPRES to Arizona and connecting it to the telescope turned out to be a combination of hair-raising and edifying.

Twice, she said, she drove from New Haven to Flagstaff with parts of the instrument; each trip in a Penske rental truck and with her son Ben helping out.

And then when the instrumentation was in process late last year, Fischer and her team learned that funds for the scientists and engineers working on that process had come to an end.

Francesco Pepe of the University of Geneva. He is the principal scientist for the ESPRESSO instrument and gave essential aid to the EXPRES team when they needed it most.

She was desperate, and sent a long-shot email to Francesco Pepe of University of Geneva, the lead scientist and wizard builder of several European spectrographs, including ESPRESSO. In theory, he and his instrument — which went into operation late last year at the ESO Very Large Telescope in Chile — will be competing with EXPRES for discoveries and acknowledgement.

Nonetheless, Pepe heard Fischer out and understood the predicament she was in.  ESPRESSO had been installed and so he was able to contact an associate who freed up two instrumentation specialists who flew to Flagstaff to finish the work.  It was, Fischer said, an act of collegial generosity and scientific largesse that she will never forget.

Fischer is at the Lowell observatory now, using the Arizona monsoon as a time to clean up many details before the team returns to full-time observing.  She write about her days in an EXPRES blog.  Earlier, in March after the instrumentation had been completed and observing had commenced, she wrote this:

“Years of work went into EXPRES and as I look at this instrument, I am surprised that I ever had the audacity to start this project. The moment of truth starts now. It will take us a few more months of collecting and analyzing data to know if we made the right design decisions and I feel both humbled and hopeful. I’m proud of the fact that our design decisions were driven by evidence gleaned from many years of experience. But did I forget anything?”

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The Architecture of Solar Systems

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The architecture of planetary systems is an increasingly important factor to exoplanet scientists.  This illustration shows the Kepler-11 system where the planets are all roughly the same size and their orbits spaced at roughly the same distances from each other.  The the planets are, in the view of scientists involved with the study, “peas in a pod.” (NASA)

Before the discovery of the first exoplanet that orbits a star like ours, 51 Pegasi b, the assumption of solar system scientists was that others planetary systems that might exist were likely to be like ours.  Small rocky planets in the inner solar system, big gas giants like Jupiter, Saturn and Neptune beyond and, back then, Pluto bringing up the rear

But 51 Peg b broke every solar system rule imaginable.  It was a giant and hot Jupiter-size planet, and it was so close to its star that it orbited in a little over four days.  Our Jupiter takes twelve years to complete an orbit.

This was the “everything we knew about solar systems is wrong” period, and twenty years later thinking about the nature and logic of solar system architecture remains very much in flux.

But progress is being made, even if the results are sometimes quite confounding. The umbrella idea is no longer that solar, or planetary, systems are pretty much like ours, but rather that the galaxy is filled with a wild diversity of both planets and planetary systems.

Detecting and trying to understand planetary systems is today an important focus 0f  exoplanet study, especially now that the Kepler Space Telescope mission has made clear that multi-planet systems are common.

As of early July, 632 multi planet systems have been detected and 2,841 stars are known to have at least one exoplanets.  Many of those stars with a singular planet may well have others yet to be found.

An intriguing newcomer to the diversity story came recently from University of Montreal astronomer Lauren Weiss, who with colleagues expanded on and studied some collected Kepler data.

What she found has been deemed the “peas in a pod” addition to the solar system menagerie.

Weiss was working with the California-Kepler Survey, which included a team of scientists pouring over, elaborating on and looking for patterns in, among other things, solar system architectures.

Weiss is part of the California-Kepler Survey team, which used the Keck Observatory to obtain high-resolution spectra of 1305 stars hosting 2025 transiting planets originally discovered by Kepler.

From these spectra, they measured precise sizes of the stars and their planets, looking for patterns in, among other things, solar system architectures.  They focused on 909 planets belonging to 355 multi-planet systems. By improving the measurements of the radii of the stars, Weiss said, they were able to recalculate the radii of all the planets.

So Weiss studied hundreds systems and did find a number of surprising, unexpected patterns.

In many systems, the planets were all roughly the same size as the planet in orbit next to them. (No tiny-Mars-to-gigantic-Jupiter transitions.)  This kind of planetary architecture was not found everywhere but it was quite common — more common than random planet sizing would predict.

“The effect showed up with smaller planets and larger ones,” Weiss told me during last week’s University of Cambridge Exoplanets2 conference. “The planets in each system seemed to know about the sizes of the neighbors,” and for thus far unknown reasons maintained those similar sizes.

What’s more, Weiss and her colleagues found that the orbits of these “planets in a pod” were generally an equal distance apart in “multi” of three planets or more. In other words, the distance between the orbits of planet A and planet B was often the same distance as between the orbits of planet B and planet C.

Lauren Weiss at the W.M Keck Observatory.

So not only were many of the planets almost the same size, but they were in orbits spaced at distances from each other that were once again much more similar than a random distribution would predict. In the Astronomical Journal article where she and her colleagues described the phenomena, they also found a “wall” defining how close together the planets orbited.

The architecture of these systems, Weiss said, reflected the shapes and sizes of the protoplanetary in which they were formed.  And it would appear that the planets had not been disrupted by larger planets that can dramatically change the structure of a solar system — as happened with Jupiter in our own.

But while those factors explain some of what was found, Weiss said other astrophysical dynamics needed to be at play as well to produce this common architecture.  The stability of the system, for instance, would be compromised if the orbits were closer than that “wall,” as the gravitational pull of the planets would send them into orbits that would ultimately result in collisions.

The improved spectra of the Kepler planets were obtained from 2011 to 2015, and the targets are mostly located between 1,000 and 4,000 light-years away from Earth.

The architectures of California-Kepler study multi-planet systems with four planets or more.  Each row corresponds to the planets around one and the circles represent the radii of planets in the system.  Note how many have lines of planets that are roughly the same size. (Lauren Weiss, The Astronomical Journal.)

Planetary system architecture was a significant topic at the Cambridge Exoplanets2 conference.  While the detection of individual exoplanets remains important in the field, it is often treated as a precursor to the ultimate detection of systems with more planets. 

The TRAPPIST-1 system, discovered in 2015 by a Belgian team, is probably the most studied and significant of those discovered so far.

The ultra-cool dwarf star hosts seven Earth-sized, temperate exoplanets in or near the “habitable zone.” As described by one of those responsible for the discovery, Brice-Olivier Demory of the Center for Space and Habitability University of Bern, the system “represents a unique setting to study the formation and evolution of terrestrial planets that formed in the same protoplanetary disk.”

The Trappist-1 architecture features not only the seven rocky planets, but also a resonance system whereby the planets orbits at paces directly related to the planets nearby them.  In other words, one planet may make two orbits in exactly the time that it takes for the next planet to make three orbits.

All the Trappist-1 planets are in resonance to another system planet, though they are not all in resonance to each other.

The animation above from the NASA Ames Research Center shows the orbits of the Trappist-1 system.  The planets pass so close to one another that gravitational interactions are significant, and to remain stable the orbital periods are nearly resonant. In the time the innermost planet completes eight orbits, the second, third, and fourth planets complete five, three, and two respectively.

The system is very flat and compact. All seven of TRAPPIST-1’s planets orbit much closer to their star than Mercury orbits the sun. Except for TRAPPIST-1b, they orbit farther than the Galilean moons — three of which are also in resonance around Jupiter.

The distance between the orbits of TRAPPIST-1b and TRAPPIST-1c is only 1.6 times the distance between the Earth and the Moon.  A year on the closest planet passes in only 1.5 Earth days, while the seventh planet’s year passes in only 18.8 days.

Given the packed nature of the system, the planets have to be in particular orbits that keep them from colliding.  But they also have to be in orbits that ensure that all or most of the planets aren’t on the same side of the star, creating a severe imbalance that would result in chaos.

“The Trappist-1 system has entered into a zone of stability,” Demory told me, also at the Exoplanets2 conference.  “We think of it as a Darwinian effect — the system survives because of that stability created through the resonance.  Without the stability, it would die. ”

He said the Trappist-1 planets were most likely formed away from their star and migrated inward.  The system had rather a long time to form, between seven and eight billion years.

The nature of some of the systems now being discovered brings to mind that early reaction to the detection of 51 Pegasi b, the world’s first known exoplanet.

The prevailing consensus that extra-solar systems would likely be similar to ours was turned on its head by the giant planet’s closeness to its host star.  For a time many astronomers thought that hot Jupiter planets would be found to be common.

But 20 years later they know that hot Jupiters — and the planetary architecture they create — are rather unusual, like the architecture of our own solar system.

With each new discovery of a planetary system, the understanding grows that while solar systems are governed by astrophysical forces, they nonetheless come in all sizes and shapes. Diversity is what binds them together.

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Exoplanet Science Flying High

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An artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. Credit: NASA/JPL-Caltech

 

Early this spring, the organizers of an exoplanet science gathering at Cambridge University put out the word that they would host a major meeting this summer.  Within a week, the 300 allotted slots had been filled by scientists aspiring and veteran, and within a short time the waiting list was up to 150 more.

Not the kind of reaction you might expect for a hardcore, topic-specific meeting, but exoplanet science is now in a phase of enormous growth and excitement.  With so many discoveries already made and waiting to be made, so many new (and long-standing) questions to be worked on, so much data coming in to be analyzed and turned into findings,  the field has something of a golden shine.

What’s more, it has more than a little of the feel of the Wild West.

Planet hunters Didier Queloz and Michel Mayor at the European Southern Observatory’s La Silla site. (L. Weinstein/Ciel et Espace Photos)

Didier Queloz, a professor now at Cambridge but in the mid 1990s half of the team that identified the first exoplanet, is the organizer of the conference.

“It sometimes seems like there’s not much exploration to be done on Earth, and the opposite is the case with exoplanets,” he told me outside the Cambridge gathering.

“I think a lot of young scientists are attracted to the excitement of exoplanets, to a field where there’s so much that isn’t known or understood.”

Michel Mayor of the Observatory of Geneva — and the senior half of the team that detected the first exoplanet orbiting a star like our sun, 51 Pegasi b– had opened the gathering with a history of the search for extra-solar planets.

That search had some conceptual success prior to the actual 1995 announcement of an exoplanet discovery, but several claims of having actually found an exoplanet had been made and shown to be wanting.  Except for the relative handful of scientists personally involved, the field was something of a sideshow.

“At the time we made our first discovery, I basically knew everyone in the field.  We were on our own.”

Now there are thousands of people, many of them young people, studying exoplanets.  And the young people, they have to be smarter, more clever, because the questions are harder.”

And enormous progress is being made.

The pace of discovery is charted here by Princeton University physicist and astronomer Joshua Winn. First is a graphic of all the 3,735 exoplanet discoveries made since 1995, and then the 1943 planets found just from 2016 to today.

The total number and distribution of known exoplanets, identified by the mass of the planet and their distance from their host star. A legend to the four major techniques for finding exoplanets is in the lower right The circled planets in green are those in our solar system. All the data comes from the NASA Exoplanet archive. (Joshua Winn, Princeton University)

 

Based on published papers, Winn found that the discovery of 1,943 new planets had been announced in papers between 2016 and today. Winn said the number is not formal as some debate remains whether a small number are planets or not.

Many of the planets discovered via the transit method come from the Kepler and K2 missions.  Kepler revolutionized the field with its four years of intensively observing a region of the sky for planet transits in front of their star.

The K2 mission began after the second of Kepler’s four stabilizing wheels failed. But adjustments were made and the second incarnation of Kepler has continued to find planets, though in a different way.

While a majority of exoplanets have been detected via the transit method, the first exoplanet was discovered by Mayor and Queloz via the radial velocity method — which involves ground-based measurements of the “wobble” of a star caused by the gravitational pull of a planet.

Many astronomers continue to use the technique because it provides more information about the minimum mass and orbital eccentricity of planet.  In addition, two high-precision, next-generation spectrometers for radial velocity measuring are now coming on line and are expected to significantly improve the detection of smaller planets using that method.

One is the ESPRESSO instrument (the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic) recently installed by the European Southern Observatory on the Very Large Telescope in Chile. The other newcomer is EXPRES, developed by scientists at Yale University, with support for the National Science Foundation.  The instrument, designed go look for Earth-sized planets, has been installed on the Lowell Observatory Discovery Channel Telescope in Arizona.

 

The Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) will search for exoplanets with unprecedented precision by looking at the minuscule changes in the properties of light coming from their host stars. This picture shows the front-end structure where the light beams coming from the four Very Large Telescopes are brought together and fed into fibers. They then deliver the photons to a spectrograph in another room, which makes the radial velocity measurements. (Giorgio Calderone, INAF Trieste)

The conference, which will go through the week, focuses both generally and in great detail on many of the core questions of the field:  how exoplanets are formed, what kind of stars are likely to produce what kinds of planets, the makeup and dynamics of exoplanet atmospheres, planet migration, the architecture of planetary systems.

And, of course, where new exoplanets might be found.  (Mostly around red dwarf stars, several scientists argued, and many in the relatively near neighborhood.)

Notably, many of the exoplanet questions being studied have clear implications for better understanding our own solar system.  In fact, it is often said that we won’t really understand the workings and history of our solar system, planets, moons, asteroids and more until we know a lot more about the billions and billion of other planetary systems in our galaxy.

Also notable for this conference is the lack of emphasis on biosignatures, habitability and the search for life beyond Earth.  The conference is billed as being about “exoplanet science,” and Queloz explained the absence of habitability and life-detection talks was based on the scientific progress made, or not made, in the past two years.

When it comes to planet detection, however, theory and practice are coming together in searches for exoplanets around smaller and cooler stars, and even around young stars where planets are just forming.  Such a planet discovery was announced this week coming from the European Space Agency’s Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument.

 

The first clear image of a planet caught while being formed,around the dwarf star PDS 70. The planet is visible as a bright point to the right of center. The star at the center is blacked out by a coronagraph mask that blocks its blinding light. The SPHERE instrument is on the European Southern Observatory’s Very Large Telescope (A. Müller et al./ESO)

 

The Cambridge exoplanet conference is the second in a series begun two years ago by Queloz and Kevin Heng, an exoplanet atmosphere theoretician at the University of Bern and director of the Center for Space and Habitability.

The two had been struck by how European exoplanet conferences seemed to be dominated by senior scientists, with little time or space for the many younger men and women coming up in the field.  The presentations also seemed more long and formal than needed.

So using funds from their own institutions to seed the conferences, Heng set up the first in Davos, Switzerland and Didier the second in Cambridge.  The idea has caught on, and similar gathering are now scheduled at two year intervals in Heidelberg, Las Vegas, Amsterdam, Porto and hopefully later in Asia, too.

There is no dearth of other exoplanet gatherings around the world, and attendees report that they are also very well attended.

But given sheer amount of work now being done in the field that was so lonely only twenty years ago,  they surely appear to be warranted.

And newsworthy, though no always reportable.

Three of the papers discussed in the Cambridge conference, for instance, are under reporting embargo from the journal Nature. And information from George Ricker, principal investigator for NASA’s Transiting Exoplanet Survey Satellite (TESS), about the early days of the mission are also under embargo.  Suffice it to say, however, that Ricker reported that things are going well for the exoplanet-hunting telescope.

 

This test image from one of the four cameras aboard the Transiting Exoplanet Survey Satellite (TESS) captures a swath of the southern sky along the plane of our galaxy. TESS is designed to study exoplanets around the brightest stars, and is expected to cover more than 400 times the amount of sky shown in this image. (NASA/MIT/TESS)

While the initial discovery of an exoplanet was difficult for sure, what the much, much larger field is grappling with now is clearly even more challenging.  With that in mind, I asked Queloz what he hoped to see from exoplanets in the years ahead.

“We have reached the point where we know stars usually have planets.  But what we are still looking for is an Earth twin — a planet clearly like ours.  That we have not found.  Before I retire, what I hope for is the discovery of that Earth twin.”

 

 

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Know Thy Star, Know Thy Planet: How Gaia is Helping Nail Down Planet Sizes

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Gaia’s all-sky view of our Milky Way and neighboring galaxies. (ESA/Gaia/DPAC)

 

 

(This column was written by my colleague Elizabeth Tasker, now at the Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Sciences (ISAS).  Trained as an astrophysicist, she researches planet and galaxy formation and also writes on space science topics.  Her book, “The Planet Factory,” came out last year.)

 

Last month, the European Space Agency’s Gaia mission released the most accurate catalogue to date of positions and motions for a staggering 1.3 billion stars.

Let’s do a few comparisons so we can be suitably amazed. The total number of stars you can see without a telescope is less than 10,000. This includes visible stars in both the northern and southern hemispheres, so looking up on a very dark night will allow you to count only about half this number.

The data just released from Gaia is accurate to 0.04 milli-arcseconds. This is a measurement of the angle on the sky, and corresponds to the width of a human hair at a distance of over 300 miles (500 km.) These results are from 22 months of observations and Gaia will ultimately whittle down the stellar positions to within 0.025 milli-arcseconds, the width of a human hair at nearly 680 miles (1000 km.)

OK, so we are now impressed. But why is knowing the precise location of stars exciting to planet hunters?

The reason is that when we claim to measure the radius or mass of a planet, we are almost always measuring the relative size compared to the star. This is true for all planets discovered via the radial velocity and transit techniques — the most common exoplanet detection methods that account for over 95% of planet discoveries.

It means that if we underestimate the star size, our true planet size may balloon from being a close match to the Earth to a giant the size of Jupiter. If this is true for many observed planets, then all our formation and evolution theories will be a mess.

The size of a star is estimated from its brightness. Brightness depends on distance, as a small, close star can appear as bright as a distant giant. Errors in the precise location of stars therefore make a big mess of exoplanet data.


An artist’s impression of the Gaia spacecraft — which is on a mission to chart a three-dimensional map of our Milky Way. In the process it will expand our understanding of the composition, formation and evolution of the galaxy. (ESA/D. Ducros)

This issue has been playing on the minds of exoplanet hunters.

In 2014, a journal paper authored by Fabienne Bastien from Vanderbilt University suggested that nearly half of the brightest stars observed by the Kepler Space Telescope are not regular stars like our sun, but actually are distant and much larger sub-giant stars. Such an error would mean planets around these stars are 20 – 30% larger than estimated, a particularly hard punch for the exoplanet community as planets around bright stars are prime targets for follow-up studies.

Previous improvements in the accuracy of the measured radii and other properties of stars have already proved their worth. In 2017, a journal paper led by Benjamin Fulton at the University of Hawaii revealed the presence of a gap in the distribution of sizes of super Earths orbiting close to their star. Planets 20% and 140% larger than the Earth appeared to be common, but there was a notable dearth of planets around twice the size of our own.

Super Earth planets with orbits of less than 100 days seem to come in two different sizes. (NASA/Ames/Caltech/University of Hawaii. (B.J.Fulton))

The most popular theory for this gap is that the peaks belong to planets with similar core sizes, but the planets with larger radii have deep atmospheres of hydrogen and helium. This would make the planets belonging to the smaller radii peak true rocky worlds, whereas the second peak would be mini Neptunes: the first evidence of a size distinction between these two regimes.

This split in the small planet population was spotted due to improved measurements of planet radii based on higher precision stellar observations made using the Keck Observatory. With a gap size of only half an Earth-radius, it had previously gone unnoticed due to the uncertainty in planet size measurements.

Both the concern of a significant error in planet sizes and the tantalizing glimpse at the insights that could be achieved with more accurate data is why Gaia is so exciting.

Launched on December 19, 2013, Gaia is a European Space Agency (ESA) space telescope for astrometry; the measurement of the position and motion of stars. The mission has the modest goal of creating a three-dimensional map of our galaxy to unprecedented precision.

Gaia measures the position of stars using a technique known as parallax, which involves looking at an object from different perspectives.

Parallax is easily demonstrated by holding up your finger and looking at it with one eye open and the other closed. Switch eyes, and you will see your finger moves in relation to the background. This movement is because you have viewed your finger from two different locations: the position of your left eye and that of your right.

Parallax is the apparent shift in the position of stars as the Earth orbits the sun. It can be used to determine distances between stars. (ESA/ATG medialab)

The degree of motion depends on the separation between your eyes and the distance to your finger: if you move your finger further from your eyes, its parallax motion will be less. By measuring the separation of your viewing locations and the amount of movement you see, the distance to an object can therefore be calculated.

Since stars are far more distant than a raised finger, we need widely separated viewing locations to detect the parallax. This can be done by observing the sky when the Earth is on opposite sides of its orbit. By measuring how far stars seem to move over a six month interval, we can calculate their distance and precisely estimate their size.

This measurement was first achieved by Friedrich Wilhelm Bessel in 1838, who calculated the distance to the star 61 Cygni. Bessel estimated the star was 10.3 light years from the Earth, just 10% lower than modern measurements which place the star at a distance of 11.4 light years.

However, measuring parallax from Earth can be challenging even with powerful telescopes. The first issue is that our atmosphere distorts light, making it difficult to measure tiny shifts in the position of more distant stars. The second problem is that the measured motion is always relative to other background stars. These more distant stars will also have a parallax motion, albeit smaller than stars closer to Earth.

As a result, the motion measured and hence the distance to a star, will depend on the parallax of the more distant stars in the same field of view. This background parallax varies over the sky, leaving no way on Earth of creating a consistent catalogue of stellar positions.

The Gaia spacecraft’s billion-pixel camera maps stars and other objects in the Milky Way. (C. Carreau/ESA)

These two conundrums are where Gaia has the advantage. Orbiting in space, Gaia simply avoids atmospheric distortion. The second issue of the background stars is tackled by a clever instrument design.

Gaia has two telescopes that point 106.4 degrees apart but project their images onto the same detector. This allows Gaia to see stars from different parts of the sky simultaneously. The telescopes slowly rotate so that each field of view is seen once by each telescope and overlaid with a field 106.4 degrees either clockwise or counter-clockwise to its position. The parallax motion of stars during Gaia’s orbit can therefore be compared both with stars in the same field of view, and with stars in two different directions.

Gaia repeats this across the sky, linking the fields of view together to globally compare stellar positions. This removes the problem of a parallax measurement depending on the motion of stars that just happen to be in the background.

The result is the relative position of all stars with respect to one another, but a reference point is needed to turn this into true distances. For this, Gaia compares the parallax motion to distant quasars.

Quasars are black holes that populate the center of galaxies and are surrounded by immensely luminous discs of gas. Being outside our Milky Way, the distance to quasars is so great that their parallax during the Earth’s orbit is negligibly small. Quasars are too rare to be within the field of view of most stars, but with stellar positions calibrated across the whole sky, Gaia can use any visible quasars to give the absolute distances to the stars.

What did these precisely measured stellar motions do to the properties of the orbiting planets? Did our small worlds vanish or the intriguing division in the sizes of super Earths disappear?

This was bravely investigated in a journal paper this month led by Travis Berger from the University of Hawaii. By matching the stars observed by Kepler to those in the Gaia catalogue, Berger confirmed that the majority of bright stars were indeed sun-like and not the suspected sub-giant population. However, the more precise stellar sizes were slightly larger on average, causing a small shift in the observed small planet radii towards bigger planets.

Planet radii derived from the new Gaia data and the Kepler (DR25) Stellar Properties Catalogue. Red points are confirmed planets while black points are planet candidates. Bottom panel shows the ratio between the two data sets. There is a small shift towards larger planets in the new Gaia data. (Figure 6 in Berger et al, 2018.)

The same result was found in a parallel study led by Fulton, who found a 0.4% increase in planet radii from Gaia compared with the (higher precision than Kepler, but less precision than Gaia) results using Keck.

The papers authored by Berger and Fulton investigated the split in super Earth sizes on short orbits, confirming that the two planet populations was still evident with the high precision Gaia data. Further exploration also revealed interesting new trends.

Fulton noticed that two peaks in the super Earth population appear at slightly larger radii for planets orbiting more massive stars. This is true irrespective of the level radiation the planets are receiving from the star, ruling out the possibility that more massive stars are simply better at evaporating away atmospheres on bigger planets. Instead, this trend implies that bigger stars build bigger planets.

Models proposed by Sheng Jin (Chinese Academy of Sciences) and Christoph Mordasini (the Max Planck Institute for Astronomy) in a paper last year proposed that the location of the split in the super Earth population could be linked to composition.

Planets made of lighter materials such as ices would need a larger size to retain their atmospheres, compared to planet cores of denser rock. If the planet size at the population split marks the transition from large rocky worlds without thick atmospheres to mini-Neptunes enveloped in gas, then it corresponds to the size needed to retain that gas.

Berger suggests that the gap between the planet populations seen in the new Gaia data is best explained by planets with an icy-rich composition. As these planets all have short orbits, this suggests these close-in worlds migrated inwards from a much colder region of the planetary system.

The high precision planet radii measurements from Gaia seem to leave our planet population intact, but suggest new trends worth exploring. This will be a great job for TESS, NASA’s recently launched planet hunter that is preparing to begin its first science run this summer. Gaia’s astrometry catalogue of stars will be ensuring we get the very best from this data.

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First Mapping of Interstellar Clouds in Three Dimensions; a Key Breakthrough for Better Understanding Star Formation

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This snakelike gas cloud (center dark area) in the constellation Musca resembles a skinny filament. But it’s actually a flat sheet that extends about 20 light-years into space away from Earth, an analysis finds.
(Dylan O’Donnell, deography.com/WikiCommons)

When thinking and talking about “astrobiology,” many people are inclined to think of alien creatures that often look rather like us, but with some kind of switcheroo.  Life, in this view, means something rather like us that just happens to live on another planet and perhaps uses different techniques to stay alive.

But as defined by NASA, and what “astrobiology” is in real scientific terms, is the search for life beyond Earth and the exploration of how life began here.  They may seem like very different subjects but are actually joined at the hip;  having a deeper understanding of how life originated on Earth is surely one of the most important set of clues to how to find it elsewhere.

Those con-joined scientific disciplines — the search for extraterrestrial life and the extraordinarily difficult task of analyzing how it started here — together raise another most complex challenge.

Precisely how far back do we look when trying to understand the origins of life?  Do we look to Darwin’s “warm little pond?” To the Miller-Urey experiment’s conclusion that organic building blocks of life can be formed by sparking some common gases and water with electricity?  To an understanding the nature and evolution of our atmosphere?

The answer is “yes” to all, as well as to scores of additional essential dynamics of our galaxies.  Because to begin to answer those three questions, we also have to know how planets form, the chemical make-up of the cosmos, how different suns effect different exoplanets and so much more.

This is why I was so interested in reading about a breakthrough approach to understanding the shape and nature of interstellar clouds.  Because it is when those clouds of gas and dust collapse under their own gravitational attraction that stars are formed — and, of course, none of the above questions have meaning without preexisting stars.

In theory, the scope of astrobiology could go back further than star-formation, but I take my lead from Mary Voytek, chief scientist for astrobiology at NASA.  The logic of star formation is part of astrobiology, she says, but the innumerable cosmological developments going back to the Big Bang are not.

So by understanding something new about interstellar clouds — in this case determining the 3D structure of such a “cloud” — we are learning about some of the very earliest questions of astrobiology, the process that led over the eons to us and most likely life of some sort on the billions of exoplanets we now know are out there.

Cepheus B, a molecular cloud located in our Milky Galaxy about 2,400 light years from the Earth, provides an excellent model to determine how stars are formed. This composite image of Cepheus B combines data from the Chandra X-ray Observatory and the Spitzer Space Telescope.The Chandra observations allowed the astronomers to pick out young stars within and near Cepheus B, identified by their strong X-ray emission.
Credits X-ray: NASA/CXC/PSU/K. Getman et al.; IRL NASA/JPL-Caltech/CfA/J. Wang et al.

So, what is an interstellar cloud?

It’s the generic name given to an accumulation of gas, plasma, and dust in our and other galaxies, left over from galaxy formation.  So an interstellar cloud is a denser-than-average region of the interstellar medium.

Hydrogen is its primary component, and that hydrogen exists in a wide variety of states depending on the density, the age, the location and more of the cloud.

Until recently the rates of reactions in interstellar clouds were expected to be very slow, with minimal products being produced due to the low temperature and density of the clouds. However, organic molecules were observed in the spectra that scientists would not have expected to find under these conditions, such as formaldehyde, methanol, and vinyl alcohol.

The reactions needed to create such substances are familiar to scientists only at the much higher temperatures and pressures of earth and earth-based laboratories. The fact that they were found indicates that these chemical reactions in interstellar clouds take place faster than suspected, likely in gas-phase reactions unfamiliar to organic chemistry as observed on earth.

What was newly revealed this week is that it is possible to determine the 3D structure of an interstellar cloud. The advance not only reveals the true structure of the molecular cloud Musca, which differs from previous assumptions in looking more like a pancake than a needle.

But the two authors, astrophysicist Konstantinos Tassis of the University of Crete and Aris Tritsis, now a postdoctoral fellow at Australian National University, say their discovery will lead to a better understanding of the evolution of interstellar clouds in general. This, in turn, which will help astronomers answer the longstanding questions of how and why the enormous number and wild variety of stars exists in our galaxy and beyond.

Here is a video put together to help explain the science of Musca and its dimensions.   The work was published in the journal Science, and here is their description of what the video shows:

“The first part of the movie gives an overview of the problem of viewing star-forming clouds in 2D projection. The second part of the video shows the striations in Musca, and the process through which the normal mode spatial frequencies are recovered. The third part of the movie demonstrates how the apparently complex profiles of the intensity cuts through striations are reproduced by progressively summing the theoretically predicted normal modes. At this part of the video (1:30-1:52) the spatial frequencies are scaled to the frequency range of human hearing and are represented by the musical crescendo.”

Credit: Aris Tritsis, Nick Gikopoulos, Valerio Calisse, Kostas Tassis

In an email, Tritsis said that this is the first time that the 3-dimensional coordinates of an interstellar cloud have been measured.

“There have been other crude estimates of the 3D sizes of clouds that relied on many assumptions so this is the first time we were able to determine the size with such accuracy and certainty,” he wrote.

“What we are after is the physics that controls the nature of the stars that will form. This physics will dictate how many star will form and with what masses, but it will also be responsible for shaping the cloud. Thus, this physics is encoded in the shape and that is why we are so interested about it.”

Their pathway in to mapping a 3D cloud was the striations (wispy stripe-like patterns) they detected within the cloud. They show that these striations form by the excitation of fast magnetosonic waves (longitudinal magnetic pressure waves) – the cloud is vibrating, like a bell ringing after it has been struck.

“What we have actually found is that the entire cloud oscillates just like waves on the surface of a pond,” he wrote in his email.

Aris Tritsis, a postdoctoral student at the Australian National University.

“However, in this instance is not the surface of the water that is oscillating but the magnetic field that is threading the cloud. Furthermore, because these waves get trapped, they act like a fingerprint. They are unique and by studying their frequencies we can deduce the sizes of the boundaries that confined them.

“It is the same concept as a violin and a cello making very different sounds. In a similar fashion, clouds with different shapes and sizes will vibrate differently. After having identify the frequencies of these oscillations we scaled them to the frequency range of human hearing to get the ‘song of Musca’!”

By analyzing the frequencies of these waves the authors produce a model of the cloud, showing that Musca is not a long, thin filament as once thought, but rather a vast sheet-like or pancake structure that stretches 20 light-years away from Earth.  (The cloud is some 27 light 490 and 650 light-years from Earth.)

With the determination of its 3D nature, the scientists modeled a cloud that is ten times more spacious than earlier thought.

Konstantinos Tassis of the University of Crete is a star formation specialist. He received his doctorate in theoretical astrophysics at the University of Illinois at Urbana Champaign.

From the 3D reconstruction, the authors were able to determine the cloud’s density. Tritsis and Tassis note that, with its geometry now determined, Musca can be used to test theoretical models of interstellar clouds.

“Because of the fact that Musca is isolated and it is very ordered, it was the obvious choice for us to test our method,” Tritis wrote. “However, other clouds out there could also vibrate globally.

“Knowing the exact dimensions of Musca, we can simulate it in great detail, calculate many different properties of this particular cloud based on different star formation models, and compare them with observations.

“We believe that, with its 3D structure revealed, Musca will now act as a prototype laboratory to study star formation in greater detail than ever before. The Musca star formation saga is only now beginning, and this is a very exciting development that goes beyond this particular discovery.”

And in that way, the discovery is very much a part of the long and broad sweep of astrobiology.

 

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