Binary star systems are downright dangerous due to their complex gravitational interactions that can easily grind a planet to pieces. So how is it that we have found a few planets in these Tattooine-like environments?
Research led by the University of Bristol show that most planets formed far away from their central stars and then migrated in at some point in their history, according to research collected concerning Kepler-34b and other exoplanets.
The scientists did “computer simulations of the early stages of planet formation around the binary stars using a sophisticated model that calculates the effect of gravity and physical collisions on and between one million planetary building blocks,” stated the university.
“They found that the majority of these planets must have formed much further away from the central binary stars and then migrated to their current location.”
You can read more about the research in Astrophysical Journal Letters. It was led by Bristol graduate student Stefan Lines with participation from advanced research fellow and computational astrophysicst Zoe Lienhardt, among other collaborators.
By now, you will probably have heard that astronomers have produced the first global weather map for a brown dwarf. (If you haven’t, you can find the story here.) May be you’ve even built the cube model or the origami balloon model of the surface of the brown dwarf Luhman 16B the researchers provided (here).
Since one of my hats is that of public information officer at the Max Planck Institute for Astronomy, where most of the map-making took place, I was involved in writing a press release about the result. But one aspect that I found particularly interesting didn’t get much coverage there. It’s that this particular bit of research is a good example of how fast-paced astronomy can be these days, and, more generally, it shows how astronomical research works. So here’s a behind-the-scenes look – a making-of, if you will – for the first brown dwarf surface map (see image on the right).
As in other sciences, if you want to be a successful astronomer, you need to do something new, and go beyond what’s been done before. That, after all, is what publishable new results are all about. Sometimes, such progress is driven by larger telescopes and more sensitive instruments becoming available. Sometimes, it’s about effort and patience, such as surveying a large number of objects and drawing conclusion from the data you’ve won.
Ingenuity plays a significant role. Think of the telescopes, instruments and analytical methods developed by astronomers as the tools in a constantly growing tool box. One way of obtaining new results is to combine these tools in new ways, or to apply them to new objects.
That’s why our opening scene is nothing special in astronomy: It shows Ian Crossfield, a post-doctoral researcher at the Max Planck Institute for Astronomy, and a number of colleagues (including institute director Thomas Henning) in early March 2013, discussing the possibility of applying one particular method of mapping stellar surfaces to a class of objects that had never been mapped in this way before.
The method is called Doppler imaging. It makes use of the fact that light from a rotating star is slightly shifted in frequency as the star rotates. As different parts of the stellar surfaces go by, whisked around by the star’s rotation, the frequency shifts vary slightly different depending on where the light-emitting region is located on the star. From these systematic variations, an approximate map of the stellar surface can be reconstructed, showing darker and brighter areas. Stars are much too distant for even the largest current telescopes to discern surface details, but in this way, a surface map can be reconstructed indirectly.
The method itself isn’t new. The basic concept was invented in the late 1950s, and the 1980s saw several applications to bright, slowly rotating stars, with astronomers using Doppler imaging to map those stars’ spots (dark patches on a stellar surface; the stellar analogue to Sun spots).
Crossfield and his colleagues were wondering: Could this method be applied to a brown dwarf – an intermediary between planet and star, more massive than a planet, but with insufficient mass for nuclear fusion to ignite in the object’s core, turning it into a star? Sadly, some quick calculations, taking into account what current telescopes and instruments can and cannot do as well as the properties of known brown dwarfs, showed that it wouldn’t work.
The available targets were too faint, and Doppler imaging needs lots of light: for one because you need to split the available light into the myriad colors of a spectrum, and also because you need to take many different rather short measurements – after all, you need to monitor how the subtle frequency shifts caused by the Doppler effect change over time.
So far, so ordinary. Most discussions of how to make observations of a completely new type probably come to the conclusion that it cannot be done – or cannot be done yet. But in this case, another driver of astronomical progress made an appearance: The discovery of new objects.
On March 11, Kevin Luhman, an astronomer at Penn State University, announced a momentous discovery: Using data from NASA’s Wide-field Infrared Survey Explorer (WISE), he had identified a system of two brown dwarfs orbiting each other. Remarkably, this system was at a distance of a mere 6.5 light-years from Earth. Only the Alpha Centauri star system and Barnard’s star are closer to Earth than that. In fact, Barnard’s star was the last time an object was discovered to be that close to our Solar system – and that discovery was made in 1916.
Modern astronomers are not known for coming up with snappy names, and the new object, which was designated WISE J104915.57-531906.1, was no exception. To be fair, this is not meant to be a real name; it’s a combination of the discovery instrument WISE with the system’s coordinates in the sky. Later, the alternative designation “Luhman 16AB” for the system was proposed, as this was the 16th binary system discovered by Kevin Luhman, with A and B denoting the binary system’s two components.
These days, the Internet gives the astronomical community immediate access to new discoveries as soon as they are announced. Many, probably most astronomers begin their working day by browsing recent submissions to astro-ph, the astrophysical section of the arXiv, an international repository of scientific papers. With a few exceptions – some journals insist on exclusive publication rights for at least a while –, this is where, in most cases, astronomers will get their first glimpse of their colleagues’ latest research papers.
Luhman posted his paper “Discovery of a Binary Brown Dwarf at 2 Parsecs from the Sun” on astro-ph on March 11. For Crossfield and his colleagues at MPIA, this was a game-changer. Suddenly, here was a brown dwarf for which Doppler imaging could conceivably work, and yield the first ever surface map of a brown dwarf.
However, it would still take the light-gathering power of one of the largest telescopes in the world to make this happen, and observation time on such telescopes is in high demand. Crossfield and his colleagues decided they needed to apply one more test before they would apply. Any object suitable for Doppler imaging will flicker ever so slightly, growing slightly brighter and darker in turn as brighter or darker surface areas rotate into view. Did Luhman 16A or 16B flicker – in astronomer-speak: did one of them, or perhaps both, show high variability?
Astronomy comes with its own time scales. Communication via the Internet is fast. But if you have a new idea, then ordinarily, you can’t just wait for night to fall and point your telescope accordingly. You need to get an observation proposal accepted, and this process takes time – typically between half a year and a year between your proposal and the actual observations. Also, applying is anything but a formality. Large facilities, like the European Southern Observatory’s Very Large Telescopes, or space telescopes like the Hubble, typically receive applications for more than 5 times the amount of observing time that is actually available.
But there’s a short-cut – a way for particularly promising or time-critical observing projects to be completed much faster. It’s known as “Director’s Discretionary Time”, as the observatory director – or a deputy – are entitled to distribute this chunk of observing time at their discretion.
On April 2, Beth Biller, another MPIA post-doc (she is now at the University of Edinburgh), applied for Director’s Discretionary Time on the MPG/ESO 2.2 m telescope at ESO’s La Silla observatory in Chile. The proposal was approved the same day.
Biller’s proposal was to study Luhman 16A and 16B with an instrument called GROND. The instrument had been developed to study the afterglows of powerful, distant explosions known as gamma ray bursts. With ordinary astronomical objects, astronomers can take their time. These objects will not change much over the few hours an astronomer makes observations, first using one filter to capture one range of wavelengths (think “light of one color”), then another filter for another wavelength range. (Astronomical images usually capture one range of wavelengths – one color – at a time. If you look at a color image, it’s usually the result of a series of observations, one color filter at a time.)
Gamma ray bursts and other transient phenomena are different. Their properties can change on a time scale of minutes, leaving no time for consecutive observations. That is why GROND allows for simultaneous observations of seven different colors.
Biller had proposed to use GROND’s unique capability for recording brightness variations for Luhman 16A and 16B in seven different colors simultaneously – a kind of measurement that had never been done before at this scale. The most simultaneous information researchers had gotten from a brown dwarf had been at two different wavelengths (work by Esther Buenzli, then at the University of Arizona’s Steward Observatory, and colleagues). Biller was going for seven. As slightly different wavelength regimes contain information about gas at slightly different colors, such measurements promised insight into the layer structure of these brown dwarfs – with different temperatures corresponding to different atmospheric layers at different heights.
For Crossfield and his colleagues – Biller among them –, such a measurement of brightness variations should also show whether or not one of the brown dwarfs was a good candidate for Doppler imaging.
As it turned out, they didn’t even have to wait that long. A group of astronomers around Michaël Gillon had pointed the small robotic telescope TRAPPIST, designed for detecting exoplanets by the brightness variations they cause when passing between their host star and an observer on Earth, to Luhman 16AB. The same day that Biller had applied for observing time, and her application been approved, the TRAPPIST group published a paper “Fast-evolving weather for the coolest of our two new substellar neighbours”, charting brightness variations for Luhman 16B.
This news caught Crossfield thousands of miles from home. Some astronomical observations do not require astronomers to leave their cozy offices – the proposal is sent to staff astronomers at one of the large telescopes, who make the observations once the conditions are right and send the data back via Internet. But other types of observations do require astronomers to travel to whatever telescope is being used – to Chile, say, to or to Hawaii.
When the brightness variations for Luhman 16B were announced, Crossfield was observing in Hawaii. He and his colleagues realized right away that, given the new results, Luhman 16B had moved from being a possible candidate for the Doppler imaging technique to being a promising one. On the flight from Hawaii back to Frankfurt, Crossfield quickly wrote an urgent observing proposal for Director’s Discretionary Time on CRIRES, a spectrograph installed on one of the 8 meter Very Large Telescopes (VLT) at ESO’s Paranal observatory in Chile, submitting his application on April 5. Five days later, the proposal was accepted.
On May 5, the giant 8 meter mirror of Antu, one of the four Unit Telescopes of the Very Large Telescope, turned towards the Southern constellation Vela (the “Sail of the Ship”). The light it collected was funneled into CRIRES, a high-resolution infrared spectrograph that is cooled down to about -200 degrees Celsius (-330 Fahrenheit) for better sensitivity.
Three and two weeks earlier, respectively, Biller’s observations had yielded rich data about the variability of both the brown dwarfs in the intended seven different wavelength bands.
At this point, no more than two months had passed between the original idea and the observations. But paraphrasing Edison’s famous quip, observational astronomy is 1% observation and 99% evaluation, as the raw data are analyzed, corrected, compared with models and inferences made about the properties of the observed objects.
For Beth Biller’s multi-wavelength monitoring of brightness variations, this took about five months. In early September, Biller and 17 coauthors, Crossfield and numerous other MPIA colleagues among them, submitted their article to the Astrophysical Journal Letters (ApJL) after some revisions, it was accepted on October 17. From October 18 onward, the results were accessible online at astro-ph, and a month later they were published on the ApJL website.
In late September, Crossfield and his colleagues had finished their Doppler imaging analysis of the CRIRES data. Results of such an analysis are never 100% certain, but the astronomers had found the most probable structure of the surface of Luhman 16B: a pattern of brighter and darker spots; clouds made of iron and other minerals drifting on hydrogen gas.
As is usual in the field, the text they submitted to the journal Nature was sent out to a referee – a scientist, who remains anonymous, and who gives recommendations to the journal’s editors whether or not a particular article should be published. Most of the time, even for an article the referee thinks should be published, he or she has some recommendations for improvement. After some revisions, Nature accepted the Crossfield et al. article in late December 2013.
With Nature, you are only allowed to publish the final, revised version on astro-ph or similar servers no less than 6 month after the publication in the journal. So while a number of colleagues will have heard about the brown dwarf map on January 9 at a session at the 223rd Meeting of the American Astronomical Society, in Washington, D.C., for the wider astronomical community, the online publication, on January 29, 2014, will have been the first glimpse of this new result. And you can bet that, seeing the brown dwarf map, a number of them will have started thinking about what else one could do. Stay tuned for the next generation of results.
And there you have it: 10 months of astronomical research, from idea to publication, resulting in the first surface map of a brown dwarf (Crossfield et al.) and the first seven-wavelength-bands-study of brightness variations of two brown dwarfs (Biller et al.). Taken together, the studies provide fascinating image of complex weather patterns on an object somewhere between a planet and a star the beginning of a new era for brown dwarf study, and an important step towards another goal: detailed surface maps of giant gas planets around other stars.
On a more personal note, this was my first ever press release to be picked up by the Weather Channel.
A recent find announced by astronomers may go a long ways towards understanding a crucial “missing link” between planets and stars.
The team, led by Friemann Assistant Professor of Physics at the University of Notre Dame’s Justin R. Crepp, recently released an image of a brown dwarf companion to a star 98 light years or 30 parsecs distant. This discovery marks the first time that a T-dwarf orbiting a Sun-like star with known radial velocity acceleration measurement has been directly imaged.
Located in the constellation Eridanus, the object weighs in at about 52 Jupiter masses, and orbits a 0.95 Sol mass star 51 Astronomical Units (AUs) distant once every 320-1900 years. Note that this wide discrepancy stems from the fact that even though we’ve been following the object for some 17 years since 1996, we’ve yet to ascertain whether we’ve caught it near apastron or periastron yet: we just haven’t been watching it long enough.
The T-dwarf, known as HD 19467 B, may become a benchmark in the study of sub-stellar mass objects that span the often murky bridge between true stars shining via nuclear fusion and ordinary high mass planets.
Brown dwarfs are classified as spectral classes M, L, T, and Y and are generally quoted as having a mass of between 13 to 80 Jupiters. Brown dwarfs utilize a portion of the proton-proton chain fusion reaction to create energy, known as deuterium burning. Low mass red dwarf stars have a mass range of 80 to 628 Jupiters or 0.75% to 60% the mass of our Sun. The Sun has just over 1,000 times Jupiter’s mass.
Researchers used data from the TaRgeting bENchmark-objects with Doppler Spectroscopy (TRENDS) high-contrast imaging survey, and backed it up with more precise measurements courtesy of the Keck observatory’s High-Resolution Echelle Spectrometer or HIRES instrument.
TRENDS uses adaptive optics, which relies on precise flexing the telescope mirror several thousands of times a second to compensate for the blurring effects of the atmosphere. Brown dwarfs shine mainly in the infrared, and objects such as HD 19467 B are hard to discern due to their close proximity to their host star. In this particular instance, for example, HD 19467 B was over 10,000 times fainter than its primary star, and located only a little over an arc second away.
“This object is old and cold and will ultimately garner much attention as one of the most well-studied and scrutinized brown dwarfs detected to date,” Crepp said in a recent Keck observatory press release. “With continued follow-up observations, we can use it as a laboratory to test theoretical atmospheric models. Eventually we want to directly image and acquire the spectrum of Earth-like planets. Then, from the spectrum, we should be able to tell what the planet is made of, what its mass is, radius, age, etc… basically all of its relevant properties.
Discovery of an Earth-sized exoplanet orbiting in a star’s habitable zone is currently the “holy grail” of exoplanet science. Direct observation also allows us to pin down those key factors, as well as obtain a spectrum of an exoplanet, where detection techniques such as radial velocity analysis only allow us to peg an upper mass limit on the unseen companion object.
This also means that several exoplanet candidates in the current tally of 1074 known worlds beyond our solar system also push into the lower end of the mass limit for substellar objects, and may in fact be low mass brown dwarfs as well.
Another key player in the discovery was the Near-Infrared Camera (second generation) or NIRC2. This camera works in concert with the adaptive optics system on the Keck II telescope to achieve images in the near infrared with a better resolution than Hubble at optical wavelengths, perfect for brown dwarf hunting. NIRC2 is most well known for its analysis of stellar regions near the supermassive black hole at the core of our galaxy, and has obtained some outstanding images of objects in our solar system as well.
What is the significance of the find? Free floating “rogue” brown dwarfs have been directly imaged before, such as the pair named WISE J104915.57-531906 which are 6.5 light years distant and were spotted last year. A lone 6.5 Jupiter mass exoplanet PSO J318.5-22 was also found last year by the PanSTARRS survey searching for brown dwarfs.
“This is the first directly imaged T-dwarf (very cold brown dwarf) for which we have dynamical information independent of its brightness and spectrum,” team lead researcher Justin Crepp told Universe Today.
Analysis of brown dwarfs is significant to exoplanet science as well.
“They serve as an essential link between our understanding of stars and planets,” Mr. Crepp said. “The colder, the better.”
And just as there has been a controversy over the past decade concerning “planethood” at the low end of the mass scale, we could easily see the debate applied to the higher end range, as objects are discovered that blur the line… perhaps, by the 23rd century, we’ll finally have a Star Trek-esque classifications scheme in place so that we can make statements such as “Captain, we’ve entered orbit around an M-class planet…”
Something that’s always been fascinating in terms of red and brown dwarf stars is also the possibility that a solitary brown dwarf closer to our solar system than Alpha Centauri could have thus far escaped detection. And no, Nibiru conspiracy theorists need not apply. Mr. Crepp notes that while possible, such an object is unlikely to have escaped detection by infrared surveys such as WISE. But what a discovery that’d be!
The bright star Fomalhaut hosts a spectacular debris disk: a dusty circling plane of small objects where planets form. At a mere 25 light-years away, we’ve been able to pinpoint detailed features: from the warm disk close by to the further disk that is comparable to the Solar System’s Kuiper belt.
But Fomalhaut never ceases to surprise us. At first we discovered a planet, Fomalhaut b, which orbits in the clearing between the two disks. Then we discovered that Fomalhaut was not a single star or a double star, but a triplet. The breaking news today, however, is that we have discovered a mini debris disk around the third star.
Fomalhaut is massive, weighing in at 1.9 times the mass of the Sun. And at such a close distance it’s one of the brightest stars in the southern sky. But its two companions are much smaller. The second star, Fomalhaut B, is 0.7 times the mass of the Sun and the third star, Fomalhaut C, a small red dwarf, is 0.2 times the mass of the Sun.
Fomalhaut C orbits Fomalhaut A at a distance of 2.5 light-years, or roughly half the distance from the Sun to the closest neighboring star. It was only confirmed to be gravitationally bound to Fomalhaut A and Fomalhaut B in October of last year.
“The disk around Fomalhaut C was a complete surprise,” lead researcher Grant Kennedy of the University of Cambridge told Universe Today. “This is only the second system in which disks around two separate stars have been discovered.”
Relatively cool dust and ice particles are much brighter at long wavelengths, allowing telescopes like the Herschel Space Telescope, to pick up the excess infrared light. However, Herschel has a much poorer resolution than an optical telescope so the image of Fomalhaut C’s disk is not spatially resolved — meaning the brightness of the disk could be measured but not its structure.
Kennedy’s team’s best guess is that the disk is quite cold, around 24 degrees Kelvin and pretty small, orbiting to and extent of 10 times the distance from the Earth to the Sun. But it’s likely that it’s similar to Fomalhaut A’s disk in that it’s bright, elliptical, and slightly offset from its host star. All three characteristics suggest that gravitational perturbations may be destabilizing the cometary orbits within the disks.
“As a stellar system Fomalhaut’s gotten very interesting in the last year,” Kennedy said. With two wide companions “it’s not obvious how the configuration came about. Forming one wide companion is not so hard, but getting a second is very unlikely. So we need to come up with a new mechanism.”
Kennedy is currently working on figuring out what exactly this “new mechanism” is and he thinks the debris disk around Fomalhaut C will provide a few helpful hints. His best guess is still under construction but it’s likely that a small star is disturbing the system.
The next step will be to watch the stellar system over the next few years in order to measure their orbits exactly. With precise motions we just might be able to see what is interrupting the system.
“We think these observations will provide a good test of the theory,” Kennedy told Universe Today. They just might “solve the mystery of why the Fomalhaut system looks like it does.”
The paper has been published in the Monthly Notices of the Royal Astronomical Society and is available for download here.
In 2012 astronomers announced the discovery of an Earth-like planet circling our nearest neighbor, Alpha Centauri B, a mere 4.3 light-years away. But with such a discovery comes heated debate. A second group of astronomers was unable to confirm the exoplanet’s presence, keeping the argument unresolved to date.
But not to worry. One need only look 2.3 light-years further to see tantalizing — although yet unconfirmed — evidence of an exoplanet circling a pair of brown dwarfs: objects that aren’t massive enough to kick-off nuclear fusion in their cores. There just may be an exoplanet in the third closest system to our Sun.
Astronomers only discovered the system last year when the brown dwarfs were spotted in data from NASA’s Wide-field Infrared Explorer (WISE). Check out a past Universe Today article on the discovery here. They escaped detection for so long because they are located in the galactic plane, an area densely populated by stars, which are far brighter than the brown dwarfs.
Henri Boffin at the European Southern Observatory led a team of astronomers on a mission to learn more about these newly found dim neighbors. The group used ESO’s Very Large Telescope (VLT) at Paranal in Chile to perform astrometry, a technique used to measure the position of the objects precisely. This crucial data would allow them to make a better estimate of the distance to the objects as well as their orbital period.
Boffin’s team was first able to constrain their masses, finding that one brown dwarf weighs in at 30 times the mass of Jupiter and the other weighs in at 50 times the mass of Jupiter. These light-weight objects orbit each other slowly, taking about 20 years.
But their orbits didn’t map out perfectly — there were slight disturbances, suggesting that something was tugging on these two brown dwarfs. The likely culprit? An exoplanet — at three times the weight of Jupiter — orbiting one or even both of the objects.
“The fact that we potentially found a planetary-mass companion around such a very nearby and binary system was a surprise,” Boffin told Universe Today.
The next step will be to monitor the system closely in order to verify the existence of a planetary-mass companion. With a full year’s worth of data it will be relatively straightforward to remove the signal caused by the exoplanet.
So far only eight exoplanets have been discovered around brown dwarfs. If confirmed, this planet will be the first to be discovered using astrometry.
“Once the companion is confirmed, this will be an ideal target to image using the upcoming SPHERE instrument on the VLT,” Boffin said. This instrument will allow astronomers to directly image planets close to their host star — a difficult technique worth the challenge as it reveals a wealth of information about the planet.
Once confirmed, this planet will stand as the closest exoplanet to the Sun, until the debate regarding Alpha Centauri Bb is resolved.
The paper has been accepted for publication as an Astronomy & Astrophysics Letter and is available for download here. For more information on Alpha Centauri Bb please read a paper available here and published in the Astrophysical Journal.
So far, just a handful of planets have been found orbiting stars in star clusters – and actually, astronomers weren’t too surprised about that. Star clusters can be pretty harsh places with hordes of stars huddling close together, with strong radiation and harsh stellar winds stripping planet-forming materials from the region.
But it turns out that perhaps astronomers are beginning to think differently about star clusters as being a homey place for exoplanets.
Scientists using several different telescopes, including the HARPS planet hunter in Chile have now discovered three planets orbiting stars in the cluster Messier 67.
“These new results show that planets in open star clusters are about as common as they are around isolated stars — but they are not easy to detect,” said Luca Pasquini from ESO, who is a co-author of a new paper about these planets. “The new results are in contrast to earlier work that failed to find cluster planets, but agrees with some other more recent observations. We are continuing to observe this cluster to find how stars with and without planets differ in mass and chemical makeup.”
The astronomers are pretty excited about one of these planets in particular, as it orbits a star that is a rare solar twin — a star that is almost identical to our Sun in all respects. This is the first “solar twin” in a cluster that has been found to have a planet.
“In the Messier 67 star cluster the stars are all about the same age and composition as the Sun,” said Anna Brucalassi from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany and lead author of the new paper on these planets. “This makes it a perfect laboratory to study how many planets form in such a crowded environment, and whether they form mostly around more massive or less massive stars.”
This cluster lies about 2,500 light-years away in the constellation of Cancer and contains about 500 stars. Many of the cluster stars are fainter than those normally targeted for exoplanet searches and trying to detect the weak signal from possible planets pushed HARPS to the limit, the team said.
They carefully monitored 88 selected stars in Messier 67 over a period of six years to look for the tiny telltale “wobbling” motions of the stars that reveal the presence of orbiting planets.
Three planets were discovered, two orbiting stars similar to the Sun and one orbiting a more massive and evolved red giant star. Two of the three planets are “hot Jupiters” — planets comparable to Jupiter in size, but much closer to their parent stars and therefore not in the habitable zone where liquid water could exist.
The first two planets both have about one third the mass of Jupiter and orbit their host stars in seven and five days respectively. The third planet takes 122 days to orbit its host and is more massive than Jupiter.
Star clusters come in two main types: open and globular. Open clusters are groups of stars that have formed together from a single cloud of gas and dust in the recent past, and are mainly found in the spiral arms of a galaxy like the Milky Way. Globular clusters are much bigger spherical collections of much older stars that orbit around the center of a galaxy. Despite careful searches, no planets have been found in a globular cluster and less than six in open clusters.
Another study last year from a team using the Kepler telescope found two planets in a dense open star cluster and the team stated that how planets can form in the hostile environments of dense star clusters is “not well understood, either observationally or theoretically.”
Exoplanets have been found in some amazing environments, and astronomers will continue to hunt for planets in these clusters of stars to try and learn more about how and why — and how many — exoplanets exist in star clusters.
If the dataset from the Kepler mission is any indication, the most common type of exoplanets in our galaxy aren’t Earth-sized rocky worlds or hot Jupiters. In fact, the most common type of exoplanet isn’t one that we see in our own neighborhood at all.
“Perhaps the most remarkable discovery by Kepler is the amount of planets between the size of Earth to four times the size of Earth,” said Geoff Marcy, professor of astronomy at University of California, speaking at the American Astronomical Society meeting this week in Washington D.C. “This is a size range that dominates the planet inventory from Kepler and it a size range not represented in our own Solar System. We don’t know for sure what these planets are made of and we don’t know how they form.”
These “mini-Neptunes” as Marcy called them, represent a huge sample in the Kepler data; about 75% of the planets found by Kepler vary in size between the Earth and Neptune, and for four years since the Kepler data have been rolling in, scientists have been trying to understand these planets.
“There’s been an enormous amount of measurements and quantitative work by the NASA Ames Kepler team,” Marcy said.
While masses and planet densities emerged from the work, astronomers still aren’t certain how they form or if they are made of rock, water or gas.
The team focused on about 42 of these planets. Two planets highlighted by Marcy in his presentation are thought to be rocky, and are named Kepler-99b and Kepler-406b. Both are forty percent larger in size than Earth and have a density similar to lead. The planets orbit their host stars in less than five and three days respectively, making these worlds too hot for life as we know it.
The team used Doppler measurements of the planets’ host stars to measure the reflex wobble of the host star, caused by the gravitational tug on the star exerted by the orbiting planet. The measured wobble reveals the mass of the planet: the higher the mass of the planet, the greater the gravitational tug on the star and hence the greater the wobble.
They also the measured transit timing variations (TTV) to determine how much neighboring planets can tug on one another causing one planet to accelerate and another planet to decelerate along its orbit.
These measurements allow for computing mass and densities of the planets, as well as figuring out the possible chemical composition of these worlds. The majority of the measurements suggest that the mini-Neptunes have a rocky core but some may have a gaseous outer shell of hydrogen or helium. Some might just be rocky with no outer envelope at all.
“What we think is happening is that some of these planets may have water on top of a rocky core,” Marcy said. “Larger planets might have the same rocky core with added gas. That’s how you get planets measuring from 1 to 4 earth radii. The planets with lower densities imply increasing amounts of gas on top of a rocky core.”
“Kepler’s primary objective is to determine the prevalence of planets of varying sizes and orbits. Of particular interest to the search for life is the prevalence of Earth-sized planets in the habitable zone,” said Natalie Batalha, Kepler mission scientist at NASA’s Ames Research Center. “But the question in the back of our minds is: are all planets the size of Earth rocky? Might some be scaled-down versions of icy Neptunes or steamy water worlds? What fraction are recognizable as kin of our rocky, terrestrial globe?”
The team said that the mass measurements produced by Doppler and TTV will help to answer these questions. The results hint that a large fraction of planets smaller than 1.5 times the radius of Earth may be comprised of the silicates, iron, nickel and magnesium that are found in the terrestrial planets here in the Solar System.
Armed with this type of information, scientists will be able to turn the fraction of stars harboring Earth-sizes planets into the fraction of stars harboring bona-fide rocky planets. And that’s a step closer to finding a habitable environment beyond the Solar System.
Marcy added later in the discussion that there’s one type of telescope that would most helpful: a Terrestrial Planet Finder type mission that would measure the temperature, size, and the orbital parameters of planets as small as our Earth in the habitable zones of distant solar systems. Alas, TPF was canceled.
The world’s newest and most powerful exoplanet imaging instrument, the recently-installed Gemini Planet Imager (GPI) on the 8-meter Gemini South telescope, has captured its first-light infrared image of an exoplanet: Beta Pictoris b, which orbits the star Beta Pictoris, the second-brightest star in the southern constellation Pictor. The planet is pretty obvious in the image above as a bright clump of pixels just to the lower right of the star in the middle (which is physically covered by a small opaque disk to block glare.) But that cluster of pixels is really a distant planet 63 light-years away and several times more massive — as well as 60% larger — than Jupiter!
And this is only the beginning.
While many exoplanets have been discovered and confirmed over the past couple of decades using various techniques, very few have actually been directly imaged. It’s extremely difficult to resolve the faint glow of a planet’s reflected light from within the brilliant glare of its star — but GPI was designed to do just that.
“Most planets that we know about to date are only known because of indirect methods that tell us a planet is there, a bit about its orbit and mass, but not much else,” said Bruce Macintosh of the Lawrence Livermore National Laboratory, who led the team that built the instrument. “With GPI we directly image planets around stars – it’s a bit like being able to dissect the system and really dive into the planet’s atmospheric makeup and characteristics.”
And GPI doesn’t just image distant Jupiter-sized exoplanets; it images them quickly.
“Even these early first-light images are almost a factor of ten better than the previous generation of instruments,” said Macintosh. ” In one minute, we were seeing planets that used to take us an hour to detect.”
Despite its large size, Beta Pictoris b is a very young planet — estimated to be less than 10 million years old (the star itself is only about 12 million.) Its presence is a testament to the ability of large planets to form rapidly and soon around newly-formed stars.
“Seeing a planet close to a star after just one minute, was a thrill, and we saw this on only the first week after the instrument was put on the telescope!” added Fredrik Rantakyro a Gemini staff scientist working on the instrument. “Imagine what it will be able to do once we tweak and completely tune its performance.”
Another of GPI’s first-light images captured light scattered by a ring of dust that surrounds the young star HR4796A , about 237 light-years away:
The left image shows shows normal light, including both the dust ring and the residual light from the central star scattered by turbulence in Earth’s atmosphere. The right image shows only polarized light. Leftover starlight is unpolarized and hence removed. The light from the back edge of the disk (to the right of the star) is strongly polarized as it reflects towards Earth, and thus it appears brighter than the forward-facing edge.
It’s thought that the reflective ring could be from a belt of asteroids or comets orbiting HR4796A, and possibly shaped (or “shepherded,” like the rings of Saturn) by as-yet unseen planets. GPI’s advanced capabilities allowed for the full circumference of the ring to be imaged.
GPI’s success in imaging previously-known systems like Beta Pictoris and HR4796A can only indicate many more exciting exoplanet discoveries to come.
“The entire exoplanet community is excited for GPI to usher in a whole new era of planet finding,” says physicist and exoplanet expert Sara Seager of the Massachusetts Institute of Technology. “Each exoplanet detection technique has its heyday. First it was the radial velocity technique (ground-based planet searches that started the whole field). Second it was the transit technique (namely Kepler). Now, it is the ‘direct imaging’ planet-finding technique’s turn to make waves.”
This year the GPI team will begin a large-scale survey, looking at 600 young stars to see what giant planets may be orbiting them.
“Some day, there will be an instrument that will look a lot like GPI, on a telescope in space. And the images and spectra that will come out of that instrument will show a little blue dot that is another Earth.”
– Bruce Macintosh, GPI team leader
The observations above were conducted last November during an “extremely trouble-free debut.” The Gemini South telescope is located near the summit of Cerro Pachon in central Chile, at an altitude of 2,722 meters.
Gas planets aren’t always bloated, monstrous worlds the size of Jupiter or Saturn (or larger) they can also apparently be just barely bigger than Earth. This was the discovery announced earlier today during the 223rd meeting of the American Astronomical Society in Washington, DC, when findings regarding the gassy (but surprisingly small) exoplanet KOI-314c were presented.
“This planet might have the same mass as Earth, but it is certainly not Earth-like,” said David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA), lead author of the discovery. “It proves that there is no clear dividing line between rocky worlds like Earth and fluffier planets like water worlds or gas giants.”
Discovered by the Kepler space telescope — ironically, during a hunt for exomoons — KOI-314c was found transiting a red dwarf star only 200 light-years away — “a stone’s throw by Kepler’s standards,” according to Kipping. (Kepler’s observation depth is about 3000 light-years.)
Kipping used a technique called transit timing variations (TTV) to study two of three exoplanets found orbiting KOI-314. Both are about 60% larger than Earth in diameter but their respective masses are very different. KOI-314b is a dense, rocky world four times the mass of Earth, while KOI-314c’s lighter, Earthlike mass indicates a planet with a thick “puffy” atmosphere… similar to what’s found on Neptune or Uranus.
Unlike those chilly worlds, though, this newfound exoplanet turns up the heat. Orbiting its star every 23 days, temperatures on KOI-314c reach 220ºF (104ºC)… too hot for water to exist in liquid form and thus too hot for life as we know it.
In fact Kipping’s team found KOI-314c to only be 30 percent denser than water, suggesting that it has a “significant atmosphere hundreds of miles thick,” likely composed of hydrogen and helium.
It’s thought that KOI-314c may have originally been a “mini-Neptune” gas planet and has since lost some of its atmosphere, boiled off by the star’s intense radiation.
Not only is KOI-314c the lightest exoplanet to have both its mass and diameter measured but it’s also a testament to the success and sensitivity of the relatively new TTV method, which is particularly useful in multiple-planet systems where the tiniest gravitational wobbles reveal the presence and details of neighboring bodies.
“We are bringing transit timing variations to maturity,” Kipping said. He added during the closing remarks of his presentation at AAS223: “It’s actually recycling the way Neptune was discovered by watching Uranus’ wobbles 150 years ago. I think it’s a method you’ll be hearing more about. We may be able to detect even the first Earth 2.0 Earth-mass/Earth-radius using this technique in the future.”
Planets are so very tiny next to stars outside of the solar system, making it really hard to spot exoplanets unless they transit across the face of their star (or if they are very, very big). Often, astronomers can only infer the existence of planets by their effect on the host star or other stars.
That’s especially true of the curious case of Kepler-88 c, which researchers using the Kepler space telescope said was a possible planet due to its effects on the orbit of Kepler-88 b, a planet that goes across the host of its host star. European astronomers just confirmed the Kepler data using the SOPHIE spectrograph at France’s Haute-Provence Observatory.
It’s the first time scientists have successfully used a technique to independently verify a planet’s mass based on what was found from the transit timing variation, or how a planet’s orbit varies from what is expected as it goes across the face of its sun. That means TTV can likely be used as a strong method on its own, advocates say.
SOPHIE’s technique relies on measuring star velocity, which also can reveal a planet’s mass by seeing its effect on the star.
“This independent confirmation is a very important contribution to the statistical analyzes of the Kepler multiple planet systems,” stated Magali Deleuil, an exoplanet researcher at Aix-Marseille University who participated in the research. “It helps to better understand the dynamical interactions and the formation of planetary systems.”
Actually, the two planets behave similarly to Earth and Mars in our own solar system in terms of orbits, according to work from a previous team (led by David Nesvorny of the Southwest Research Institute). They predicted the planets have a two-to-one resonance, which is approximately true of our own solar system since Mars takes about two Earth years to orbit the sun.