Exoplanets

Exiled exoplanet possibly kicked out of star's local neighborhood

Berkeley — A planet discovered last year sitting at an unusually large distance from its star – 16 times farther than Pluto is from the sun – may have been kicked out of its birthplace close to the star in a process similar to what may have happened early in our own solar system’s history. 

Images from the Gemini Planet Imager (GPI) in the Chilean Andes and the Hubble Space Telescope show that the star has a lopsided comet belt indicative of a very disturbed solar system, and hinting that the planet interactions that roiled the comets closer to the star might have sent the exoplanet into exile as well.

The planet may even have its own ring of debris it dragged along with it when it was expelled.

“We think that the planet itself could have captured material from the comet belt, and that the planet is surrounded by a large dust ring or dust shroud,” said Paul Kalas, an adjunct professor of astronomy at UC Berkeley. “We conducted three tests and found tentative evidence for a dust cloud, but the jury is still out.”

“The measurements we made on the planet suggest it may be dustier than comparison objects, and we are making follow-up observations to check if the planet is really encircled by a disk – an exciting possibility,” said Abhi Rajan, a graduate student at Arizona State University who analyzed the planet images.

Such planets are of interest because in its youth, our own solar system may have had planets that were kicked out of the local neighborhood and are no longer among of the eight planets we see today.

“Is this a picture of our solar system when it was 13 million years old?” asks Kalas. “We know that our own belt of comets, the Kuiper belt, lost a large fraction of its mass as it evolved, but we don’t have a time machine to go back and see how it was decimated. One of the ways, though, is to study these violent episodes of gravitational disturbance around other young stars that kick out many objects, including planets.”

The disturbance could have been caused by a passing star that perturbed the inner planets, or a second massive planet in the system. The GPI team also looked for another large planet closer to the star that may have interacted with the exoplanet, but found nothing outside of a Uranus-sized orbit.
 
Kalas and Rajan will discuss the observations during a Google+ Hangout On Air at 7 a.m. Hawaii time (noon EST) on Dec. 1 during Extreme Solar Systems III, the third in a series of international meetings on exoplanets that this year takes place on the 20 anniversary of the discovery of the first exoplanet in 1995. Viewers without Google+ accounts may participate via YouTube.

A paper about the results, with Kalas as lead author, was published in the The Astrophysical Journal on November 20, 2015.

Young, 13-million-year-old star

The star, HD 106906, is located 300 light years away in the direction of the constellation Crux and is similar to the sun, but much younger: about 13 million years old, compared to our sun’s 4.5 billion years. Planets are thought to form early in a stars history, however, and in 2014 a team led by Vanessa Bailey at the University of Arizona discovered a planet HD 106906 b around the star weighing a hefty 11 times Jupiter’s mass and located in the star’s distant suburbs, an astounding 650 AU from the star (one AU is the average distance between Earth and the sun, or 93 million miles).

Planets were not thought to form so far from their star and its surrounding protoplanetary disk, so some suggested that the planet formed much like a star, by condensing from its own swirling cloud of gas and dust. The GPI and Hubble discovery of a lopsided comet belt and possible ring around the planet points instead to a normal formation within the debris disk around the star, but a violent episode that forced it into a more distant orbit.

Kalas and a multi-institutional team using GPI first targeted the star in search of other planets in May 2015 and discovered that it was surrounded by a ring of dusty material very close to the size of our own solar system’s Kuiper Belt. The emptiness of the central region – an area about 50 AU in radius, slightly larger than the region occupied by planets in our solar system – indicates that a planetary system has formed there, Kalas said. 

He immediately reanalyzed existing images of the star taken earlier by the Hubble Space Telescope and discovered that the ring of dusty material extended much farther away and was extremely lopsided. On the side facing the planet, the dusty material was vertically thin and spanned nearly the huge distance to the known planet, but on the opposite side the dusty material was vertically thick and truncated.

“These discoveries suggest that the entire planetary system has been recently jostled by an unknown perturbation to its current asymmetric state,” he said. The planet is also unusual in that its orbit is possibly tilted 21 degrees away from the plane of the inner planetary system, whereas most planets typically lie close to a common plane.

Kalas and collaborators hypothesized that the planet may have originated from a position closer to the comet belt, and may have captured dusty material that still orbits the planet. To test the hypothesis, they carefully analyzed the GPI and Hubble observations, revealing three properties about the planet consistent with a large dusty ring or shroud surrounding it. However, for each of the three properties, alternate explanations are possible.

The investigators will be pursuing more sensitive observations with the Hubble Space Telescope to determine if HD 106906b is in fact one of the first exoplanets that resembles Saturn and its ring system.

The inner belt of dust around the star has been confirmed by an independent team using the planet-finding instrument SPHERE on the ESO’s Very Large Telescope in Chile. The lopsided nature of the debris disk was not evident, however, until Kalas called up archival images from Hubble’s Advanced Camera for Surveys.

The GPI Exoplanet Survey, operated by a team of astronomers at UC Berkeley and 23 other institutions, is targeting 600 young stars, all less than approximately 100 million years old, to understand how planetary systems evolve over time and what planetary dynamics could shape the final arrangement of planets like we see in our solar system today.

Among Kalas’s coauthors are UC Berkeley graduate student Jason Wang and ASU graduate student Abhijath Rajan. The research was supported by the NSF and NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate.

Research Thrust: 

Gemini Planet Imager Exoplanet Survey -- One Year Into The Survey

GPI

The Gemini Planet Imager Exoplanet Survey (GPIES) is an ambitious three-year study dedicated to imaging young Jupiters and debris disks around nearby stars using the GPI instrument installed on the Gemini South telescope in Chile. On November 12, at the 47th annual meeting of the AAS’s Division for Planetary Sciences in Washington DC, Franck Marchis, Chair of the Exoplanet Research Thrust of the SETI Institute and a scientist involved in the project since 2004, will report on the status of the survey, emphasizing some discoveries made in its first year.

Led by Bruce Macintosh from Stanford University, the survey began a year ago and has already been highly successful, with several findings already published in peer-reviewed journals.

“This very large survey is observing 600 young stars to look for two things: giant planets orbiting them and debris disks. In our first year, we have already found what GPI was designed to discover -- a young Jupiter in orbit around a nearby star,” said Marchis. This discovery was announced in an article published in Science on Oct. 2, 2015 [http://www.sciencemag.org/content/350/6256/64], with an impressive list of 88 co-authors from 39 institutions located in North and South America. “This is modern astronomy at its best,” said Marchis. “These large projects gather energy and creativity from many groups of researchers at various institutions, enabling them to consider different strategies to improve the on-sky efficiency of the instrument and its scientific output.”

The survey was officially launched in November 2014. Eight observing runs allowed the study of approximately 160 targets, or a quarter of the sample. Other parts of the survey are more frustrating, though. Due to the incipient El Nino, weather in Chile is worse than expected, with clouds, rain, snow, and atmospheric turbulence too severe even for GPI to fix. Since late June, out of the last 20 nights that team members have spent at the telescope, they’ve only gotten a few hours of good quality data Despite this loss, over which the team of course had no control, they have already published ten peer-reviewed papers in the last year. Two of the findings are described below.

GPI data has revealed that 51 Eri b, the recently discovered Jupiter-like exoplanet around the nearby star 51 Eridani [http://www.gemini.edu/node/12403], indeed has an atmosphere of methane and water, and likely has a mass twice that of Jupiter. The team has continued to observe this planetary system, and observations recorded on Sept. 1, 2015, are most consistent with a planet orbiting 51 Eri and not a brown dwarf passing along our line of sight.

“Thanks to GPI’s incredible precision, we can demonstrate that the odds are vanishingly small that 51 Eri b is actually a brown dwarf that has a chance alignment with this star. In fact it’s five times more likely that I’ll be struck by lightning this year than future data will show this is not a planet orbiting 51 Eri” said Eric Nielsen, a postdoctoral scholar at the SETI Institute and one of the authors of the paper recently accepted for publication in the Astrophysical Journal Letters [http://arxiv.org/abs/1509.07514]. Another author of this study, SETI Research Experience for Undergraduates student Sarah Blunt, analyzed the motion of 51 Eri b and found it to be completely consistent with a planet on an approximately 40-year orbit around its host star.

The team has also discovered and imaged disks of dusty debris around several stars. Astronomers believe that these are planetary systems that are still forming their planets. Some have complex structures because they host planets and fragments of the asteroidal and cometary materials that formed those planets. One such system is HD 131835: a massive 15 Myr-old star located 400 light-years from Earth. Using GPI’s high-contrast capability, the team imaged this disk for the first time in near-infrared light in May 2015.

“The disk shows different morphology when observed in different wavelengths. Unlike the extended disk previously imaged in thermal emission, our GPI observations show a disk that has a ring-like structure, indicating that the large grains are distributed differently from the small ones. In addition, we discovered an asymmetry in the disk along its major axis. What causes this disk to be asymmetric is the subject of ongoing investigation, “ said Li-Wei Hung, a graduate student in the UCLA Department of Physics and Astronomy and lead author of the article accepted to the Astrophysical Journal Astrophysical Journal Letters [http://arxiv.org/abs/1511.06767]. As asymmetries like the one seen in the system may be due to the gravitational influence of an unseen planet, more detailed observational study could one day confirm its existence.

As the GPIES survey enters in its second year, we are collaborating with the Gemini Observatory to continue to improve the instrument. The Gemini South telescope primary mirror was recently re-coated with silver to improve reflectivity, and the GPI instrument was equipped with a new cooling system to optimize performance.

“Continued collaboration between the Gemini Observatory and the GPIES collaboration has worked really well -- we’re learning a lot about how it performs in the field and interacts with the atmosphere, and are working to make GPI an even a better instrument to see even fainter and closer planets,” said Bruce Macintosh, principal investigator of the project and professor at Stanford University.

Research Thrust: 

WILLIAM BORUCKI RECEIVES SETI INSTITUTE’S 2015 DRAKE AWARD

Bill Borucki

MOUNTAIN VIEW — On October 15, the SETI Institute will award the 2015 Frank Drake Award for Innovation in SETI and Astrobiology Research to William Borucki, who was the Principal Investigator for NASA’s Kepler mission.  

Borucki will be honored for leading a team that conceived, designed, built and operated a space telescope that has detected dozens of possible Earth-size worlds situated in the habitable zone, the range of distances from a star where liquid water might exist on a planet’s surface.  Fifteen of these worlds have been verified and double that number await confirmation, although scientists do not know if they harbor life.

“The Frank Drake Award for Innovation in SETI and Life in the Universe Research honors distinguished contributors to the search for life beyond Earth,” said Bill Diamond, President and CEO of the SETI Institute.  “Awardees are chosen by a panel of scientists who are appointed by the SETI Institute Board of Trustees.  There are few developments more important to the quest for life beyond Earth than the new worlds discovered by the Kepler mission.”

In total, the Kepler mission has so far discovered more than one thousand confirmed exoplanets, and 3,600 additional candidates are awaiting verification.  More than half of all the known exoplanets have been found by this spacecraft.

Borucki is the third recipient of the Drake Award, which was launched in 2001 with a ceremony honoring its namesake.  Frank Drake conducted a pioneering search for signals from extraterrestrial intelligence in 1960, and a year later developed a simple equation that can be used to estimate the prevalence of technically sophisticated societies in the Milky Way.  

The award was also given to Charles Townes, a physicist who won the Nobel Prize for his work on developing the first masers and lasers.  Townes was also a champion of so-called optical SETI, a scheme in which mirror-and-lens telescopes are used to hunt for brief pulses of light that could be signals from other worlds.

“Kepler has been hugely successful in determining that there must be several billion terrestrial planets in the habitable zone of solar-like stars,” says Borucki.  “The knowledge that our galaxy is filled with planets, but that the SETI Program has not heard from anyone is intriguing; so which factor in the well-known Drake equation accounts for the missing communications?  Clearly, the continuation of the search is critical to understanding mankind’s position in the universe.”

Borucki, who has been a space scientist at NASA’s Ames Research Center since 1962, has degrees in physics from the University of Wisconsin, Madison, as well as a masters in meteorology from San Jose State University.  His early research helped to design the heat shields for the Apollo Mission re-entry vehicles.  He also measured lightning activity on other planets.

In the early 1980s, Borucki began to study the feasibility of detecting exoplanets using the so-called transit method.  This requires sensing the very slight (typically 0.01 percent for an Earth-size planet) dimming of a star that occurs as planet passes in front of it. Eventually, Borucki concluded that it would be possible to measure such slight dimming with an orbiting telescope. After years of persistent application to NASA, the Kepler spacecraft – designed to survey 150,000 stars – was launched in 2009.  It is arguably one of the most successful astronomical instruments of all time, and clearly relevant to SETI research because it addresses the question of what fraction of stars has habitable planets.

The award will be presented at a private function hosted by the SETI Institute.

Research Thrust: 

Watching an Exoplanet in Motion Around a Distant Star

A note by Franck Marchis, Communications Lead and EPO of GPI and the Exoplanets Research Thrust Chair.

GPI
The Gemini Planet Imager on the Gemini South telescope. In the photo, GPI comprises the three box-like components attached to the telescope and hanging closest to the observatory floor. Other box-like components on the telescope are other instruments. Image: Manuel Paredes/Gemini Observatory/AURA.

In a major breakthrough for exoplanet discovery and exploration, the Gemini Planet Imager (GPI) is proving to be one of most powerful and effective instruments ever invented for directly imaging planets in orbit around other stars.

The behind-the-scenes story of this project sheds light on the complexities and challenges of designing and building a truly game-changing instrument. We started work more than thirteen years ago under the leadership of Bruce Macintosh and the auspices of the Center for Adaptive Optics. At that time, a number of scientists, most from California and Canada, met to discuss building a groundbreaking adaptive optics (AO) system powerful enough to confront -- and overcome -- the challenging of directly collecting photons from young Jupiter-like exoplanets. The discovery of 51 Eri b (link here), which was announced last month, is the culmination of that effort.

Today, GPI is fully operational and in the process of observing ~600 bright and nearby stars as part of an 900h-survey to search for exoplanets and their circumstellar disks. There's no way to predict how many we will find, but the survey has already generated interesting and groundbreaking data -- allowing scientists, for example, to study exoplanet Beta Pictoris b. The little white dot moving in the video below is the faint glow of this young and warm exoplanet, which is 60 light years away and was observed nine times by GPI between November 2013 and April 2015. Collecting an image of this planet and its star allows us to infer the orbit and composition of the planet, and measure its temperature. In time, GPI and its successor instruments will allow us to do far more.

Images like these make us confident that another revolution in human understanding of the cosmos has begun. Twentieth-century astronomers fundamentally changed our understanding of the universe and our place in it when they mapped asteroids, comets and satellites in our solar system and beyond. Their twenty-first century counterparts have begun work on what may well be an even greater revolution by expanding our knowledge of the Milky Way and mapping far more distant objects such as stars and exoplanets.

We've just begun work on this difficult but revolutionary task. GPI and the next generation of ground- and space-based telescopes equipped with advanced AO technology are the key to finding earth-like exoworlds, including ones that other forms of life may call home.

Read the Press release below

beta pictoris b
This artist’s view shows the planet orbiting the young star Beta Pictoris. Credit: ESO L. Calçada/N. Risinger (skysurvey.org)

TORONTO - A team of astronomers has given us our best view yet of an exoplanet moving in its orbit around a distant star. A series of images captured between November 2013 to April 2015 shows the exoplanet β Pic b as it moves through 1 ½ years of its 22-year orbital period.

First discovered in 2008, β Pic b is a gas giant planet ten to twelve times the mass of Jupiter, with an orbit roughly the diameter of Saturn’s. It is part of the dynamic and complex system of the star β Pictoris which lies over 60 light-years from Earth. The system includes comets, orbiting gas clouds, and an enormous debris disk that in our Solar System would extend from Neptune’s orbit to nearly two thousand times the Sun/Earth distance.

Because the planet and debris disk interact gravitationally, the system provides astronomers with an ideal laboratory to test theories on the formation of planetary systems beyond ours.

Maxwell Millar-Blanchaer, a PhD-candidate in the Department of Astronomy & Astrophysics, University of Toronto, is lead author of a paper to be published September 16th in the Astrophysical Journal. The paper describes observations of the β Pictoris system made with the Gemini Planet Imager (GPI) instrument on the Gemini South telescope in Chile.

“The images in the series represent the most accurate measurements of the planet’s position ever made,” says Millar-Blanchaer. “In addition, with GPI, we’re able to see both the disk and the planet at the exact same time. With our combined knowledge of the disk and the planet we’re really able to get a sense of the planetary system’s architecture and how everything interacts.”

The paper includes refinements to measurements of the exoplanet’s orbit and the ring of material circling the star which shed light on the dynamic relationship between the two. It also includes the most accurate measurement of the mass of β Pictoris to date and shows it is very unlikely that β Pic b will pass directly between us and its parent star.

Astronomers have discovered nearly two thousand exoplanets in the past two decades but most have been detected with instruments—like the Kepler space telescope—that use the transit method of detection: astronomers detect a faint drop in a star’s brightness as an exoplanet transits or passes between us and the star, but do not see the exoplanet itself.

With GPI, astronomers image the actual planet—a remarkable feat given that an orbiting world typically appears a million times fainter than its parent star. This is possible because GPI’s adaptive optics sharpen the image of the target star by cancelling out the distortion caused by the Earth’s atmosphere; it then blocks the bright image of the star with a device called a coronagraph, revealing the exoplanet.

Laurent Pueyo is with the Space Telescope Science Institute and a co-author on the paper. “It’s fortunate that we caught β Pic b just as it was heading back—as seen from our vantage point—toward β Pictoris,” says Pueyo. “This means we can make more observations before it gets too close to its parent star and that will allow us to measure its orbit even more precisely.”

GPI is a groundbreaking instrument that was developed by an international team led by Stanford University’s Prof. Bruce Macintosh (a U of T alumnus) and the University of California Berkeley’s Prof. James Graham (former director of the Dunlap Institute for Astronomy & Astrophysics, U of T).

In August 2015, the team announced its first exoplanet discovery: a young Jupiter-like exoplanet designated 51 Eri b. It is the first exoplanet to be discovered as part of the GPI Exoplanet Survey (GPIES) which will target 600 stars over the next three years.

 

Research Thrust: 

NEW INSTRUMENT IMAGES A YOUNG COUSIN OF JUPITER

An artistic conception of the Jupiter-like exoplanet, 51 Eri b, seen in the near-infrared light that shows the hot layers deep in its atmosphere glowing through clouds. Because of its young age, this young cousin of our own Jupiter is still hot and carries information on the way it was formed 20 million years ago. Download fullsize credits: Danielle Futselaar & Franck Marchis, SETI Institute

MOUNTAIN VIEW – Using a powerful new imaging device, astronomers have espied a Jupiter-like exoplanet 100 light-years distant in the constellation of Eridanus.  Unlike most planets found around other stars, 51 Eri b has been seen directly.  The instrument employed to make the discovery has also made a spectroscopic analysis of the light reflected from the planet, and has detected gases similar to those in Jupiter’s atmosphere.

51 Eri Location of 51 Eri in the constellation of Eridanus (The River). With a brightness of 5 magnitude in visible and a declination of -2 degrees, the star is visible with naked eyes in the Northern and Southern hemispheres. Download fullsize credit: Astrostudio.com & Sarah Blunt, SETI Institute

“This is the first exoplanet discovered with the Gemini Planet Imager (GPI), one of the new generation instruments designed specifically for discovering and analyzing faint, young planets orbiting bright stars,” says Franck Marchis, Senior Planetary Astronomer at the SETI Institute and member of the team that built the instrument and now conducts the survey.  “GPI is far more sensitive than its predecessors.  In fact, the 51 Eri system had been observed by four previous-generation instruments that all missed the planet completely.”  

The host star, 51 Eri, is very young, a mere 20 million years old, and is slightly hotter than the Sun. The exoplanet 51 Eri b, whose mass is estimated to be roughly twice that of Jupiter, appears to orbit its host star at a distance 13 times greater than the Earth-Sun distance.  If placed in our own solar system, 51 Eri b’s orbit would lie between those of Saturn and Neptune.

“51 Eri has everything we’re looking for in a target star,” notes Eric Nielsen, a postdoctoral fellow at the SETI Institute.  “It’s relatively close and young.  Indeed, the last dinosaur died 40 million years before this star was even born.”

Because GPI not only images exoplanets but also spreads their light for chemical analysis, astronomers can search for such common gases as water and methane in their atmospheres.  Researchers had expected to see methane in directly-imaged exoplanets based on the temperature and chemistry of these worlds, but had failed to detect these molecules in large quantities using earlier instruments.  However, the observations of 51 Eri b made with GPI have clearly revealed a methane-dominated atmosphere similar to that of Jupiter.

An extraordinarily complex instrument the size of a small car, GPI is attached to one of the world’s biggest telescopes – the 8-meter Gemini South instrument in Chile.  It began its survey of stars last year.

51 Eri Discovery image of the planet 51 Eridani b with the Gemini Planet Imager taken in the near-infrared light on December 18 2014. The bright central star has been mostly removed to enable the detection of the exoplanet one million times fainter. Download fullsize Credit: J. Rameau (UdeM) and C. Marois (NRC Herzberg)

“This is exactly the kind of planet we envisioned discovering when we designed GPI,” says James Graham, a professor at the University of California, Berkeley and Project Scientist for GPI.

Astronomers anticipate that 51 Eri b will be a benchmark for future atmospheric studies that seek to understand how planet formation in these extrasolar systems might be similar to the birth of the gas giants in our own system.

“Any planetary astronomer that inspects our data will conclude without the need of complex computer modeling that this is indeed a planet like our own Jupiter. We have found its first distant and younger cousin,” said Marchis.

"51 Eri b is the first one that’s cold enough and close enough to the star that it could have indeed formed the same way Jupiter did,” adds Bruce Macintosh, who spearheaded the construction of GPI and now heads up the survey. “This whole planetary system could be a lot like ours.”

Marchis, Nielsen and the team members published their results in the Aug. 13 issue of Science or on ArXiv.

Research Thrust: 

NEW KEPLER EXOPLANET CATALOG INCLUDES TERRESTRIAL SIZED WORLD ORBITING COUSIN OF THE SUN

Kepler 452bArtist impression of the surface of Kepler 452b. With a radius 60% larger than Earth, the planet has a better than even chance of having a rocky composition, and is likely to have a thick atmosphere and a significant amount of water. Because we can’t yet measure the mass of Kepler 452b, astronomers rely on models to estimate a range of possible masses, with the most likely being 5 times that of Earth. Such a massive rocky planet would likely still have active volcanism. Kepler 452b is orbiting a close cousin of our Sun, but one that is 1.5 billion years older.  In the scenario depicted here, the planet is just entering a runaway greenhouse phase of its climate history.  The increasing energy from its aging sun could be evaporating any oceans, leaving behind large lakes ringed with mineral deposits. Kepler 452b could be giving us a preview of what the Earth will undergo more than a billion years from now, as the Sun ages and grows brighter. Credit: SETI Institute/Danielle Futselaar

MOUNTAIN VIEW – Scientists analyzing four years of data from NASA’s Kepler mission have released a new catalog of exoplanet candidates.  The catalog adds more than 500 new possible planets to the 4,175 already found by the famed space-based telescope. 

“This catalog contains our first analysis of all Kepler data, as well as an automated assessment of these results,” says SETI Institute scientist Jeffrey Coughlin who led the catalog effort.  “Improved analysis will allow astronomers to better determine the number of small, cool planets that are the best candidates for hosting life.”

The Kepler space telescope identifies possible planets by observing periodic dips in the brightness of stars.  However, confirmation of their true planetary status requires observations by other instruments, typically looking for slight shifts in the motion of the host suns.  Historically, the overwhelming majority of Kepler’s discoveries have turned out to be actual planets. 

The new catalog includes 12 candidates that are less than twice Earth’s diameter, orbiting in the so-called habitable zone of their star.  This zone is the range of distances at which the energy flux from the star would permit liquid water to exist on the planet’s surface.  Of these candidates, Kepler 452b is the first to be confirmed as a planet.  At a distance of 1400 light-years, Kepler 452b accompanies a star whose characteristics are very similar to the Sun: it is 4 percent more massive and 10 percent brighter.  Kepler 452b orbits its star at the same distance as Earth orbits the Sun.

“Kepler 452b takes us one step closer to understanding how many habitable planets are out there,” notes Joseph Twicken, also of the SETI Institute and the lead scientific programmer for the Kepler mission.  “Continued investigation of the other candidates in this catalog and one final run of the Kepler science pipeline will help us find the smallest and coolest planets.  Doing so will allow us to better gauge the prevalence of habitable worlds.”

kepler 452b comparisonThis size and scale of the Kepler-452 system compared alongside the Kepler-186 system and the solar system. Kepler-186 is a miniature solar system that would fit entirely inside the orbit of Mercury. Credits: NASA/JPL-CalTech/R. Hurt

Kepler 452b has a better than even chance of being a rocky world on the basis of its size and the type of star that it orbits.  It falls into a class of planets that are between the size of Earth and Neptune.  While these are the most abundant type of world found by Kepler, our own solar system does not boast such a planet.

Intriguingly, while similar in size and brightness to the Sun, Kepler 452b’s host star is 1.5 billion years older.  It therefore can give us a peek into a crystal ball showing a possible future for Earth. 

“If Kepler 452b is indeed a rocky planet, its location vis a vis its star could mean that it is just entering a runaway greenhouse phase of its climate history,” says Doug Caldwell, a SETI Institute scientist working on the Kepler mission.  “The increasing energy from its aging sun might be heating the surface and evaporating any oceans.  The water vapor would be lost from the planet forever.”

“Kepler 452b could be experiencing now what the Earth will undergo more than a billion years from now, as the Sun ages and grows brighter.”

The discovery of Kepler 452b will appear in the Astronomical Journal.  The latest Kepler candidate catalog can be found in the NASA Exoplanet Archive (exoplanetarchive.ipac.caltech.edu).

Researchers from NASA’s Ames Research Center, McDonald Observatory, the Harvard-Smithsonian Center for Astrophysics, the University of California, Berkeley, the NASA Exoplanet Science Institute, the University of California, Santa Cruz, and Penn State University were also involved in this work.

Research Thrust: 

DISCOVERY OF A MARS-SIZE WORLD USES TUG-OF-WAR TECHNIQUE

TODAY AT 1PM PDT Join scientists from the SETI Institute and Penn State at 1 pm PDT/4 pm EDT on a Hangout about the Kepler Space Telescope and Kepler 138b. Guests include Jason Rowe, Nathalie Cabrol, and Daniel Jontof-Hutter. Hosted by Jill Tarter.
https://plus.google.com/events/cotln5315okuo0reut5m7nbvbc8

MOUNTAIN VIEW – With a remarkable scheme that uses the motions of one planet to find another, astronomers have discovered a Mars-sized world 200 light-years from Earth. The discovery is equivalent to sensing a pin head at the distance of the Moon. 

The planet, Kepler 138b, is one of the smallest exoplanet yet found.  It orbits a diminutive red dwarf star in the northern hemisphere constellation of Lyra.

“We’ve been able to tease out both the size and mass of this planet, says SETI Institute astronomer Jason Rowe, one of the Kepler Mission team members making the discovery.  “Therefore, we know its average density, and find that it is consistent with Mars in our own Solar System.”

It didn’t have to be that way, Rowe notes.  “Exoplanets come in all sizes and masses.  Some are rocky spheres like Venus, Earth, and Mars, but others are puffed up gas balls.  So it’s interesting to see that, at least in this case, a Mars-size planet could really be Mars-like.” 

Kepler 138BKepler-138 b: the first exoplanet smaller than Earth with both it mass and size measured. All three transiting exoplanets of Kepler-138 have been characterized by their transits as they pass in front of the star each orbit. In this artist's conception, the sizes of the planets relative to the star have been exaggerated. Kepler-138 b, in the foreground is likely an airless rocky world. Because of the gravitational forces between the planets, they are slightly removed from the positions that would be expected without their mutual tugs. This enabled their masses to be measured. Download full size Illustration credit: Danielle Futselaar

The technique used to find Kepler 138b harkens back to the historic discovery of Neptune.  In the early 19th century, careful observations of the planet Uranus had showed slight irregularities in its orbital motions.  Astronomers of the time suspected these might be due to the tug of another world somewhat farther from the Sun.  That world, finally found in 1846, was Neptune.

A similar technique was used to make this new discovery.  Kepler 138 has three known planets, sensed by the Kepler space telescope when they pass in front of the star and dim its light in a mini-eclipse, or “transit.”  Study of the two planets that are exterior to Kepler 138b show that their transits sometimes occur an hour early or an hour later.  This lack of planetary punctuality is caused by the gravitational tug of their innermost neighbor.  

Once it was recognized that planetary interaction was causing the variation in transit times, straightforward application of Newton’s laws provided a mass for Kepler 138b.  Its size had already been gauged from the amount of light it blocked when transiting its star.  Combining mass and size yields a value for average density, one that is typical for a rocky world. 

Kepler 138b may be Mars-like, but it’s unlikely to be an environment favored with biology.  It’s a world in a tight orbit, and even though the red dwarf star it orbits is cooler than the Sun, the temperatures on Kepler 138 b are going to be 400 – 500K, or similar to the inside of an oven set to “bake”.  Another problem is that this world is probably tidally locked, which is to say that one side may always face its star, exacerbating the extreme temperatures. 

The technique of using transit timing to measure the tug-of-war within tightly knit planetary systems promises to be extraordinarily fruitful.  If future instruments can make transit measurements extending over a decade or more, it will be possible to determine the masses of exoplanets far more accurately than any other technique being used today.

A novel corollary of interacting planets is that they are, quite obviously, endlessly perturbing one another’s orbital period. 

“Any extraterrestrials living in these systems would have to be expert watch makers,” says Rowe, “because the length of their year is always changing.  In this case by hours, but in some others, by days.” 

The research will appear in the journal Nature. Researchers from NASA's Ames Research Center, Penn State University, and the University of Chicago were also involved in this work.

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