A Solar System Found Crowded With Seven Earth-Sized Exoplanets

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Seven Earth-sized rocked planets have been detected around the red dwarf star TRAPPIST-1. The system is 40 light years away, but is considered to be an easy system to study — as explanet research goes. (NASA)

Seven planets orbiting one star.  All of them roughly the size of Earth.  A record three in what is considered the habitable zone, the distance from the host star where liquid water could exist on the surface.  The system a mere 40 light-years away.

The latest impressive additions to the world of exoplanets orbit the dwarf star known as TRAPPIST-1, named after a European Southern Observatory telescope in Chile.

Previously a team of astronomers based in Belgium discovered three  planets around this dim star, but now that number has increased to include the largest number of Earth-sized planets found to date, as well as the largest number in one solar system in the habitable zone.

This is a very different kind of sun-and-exoplanet system than has generally been studied.  The broad quest for an Earth-sized planet in a habitable zone has focused on stars of the size and power of our sun.  But this one is 8 percent the mass of our sun —  not that much larger than Jupiter — and with a luminosity (or energy) but 0.05 percent of that put out by our sun.

The TRAPPIST-1 findings underscore one of the recurring and intriguing aspects of the exoplanet discoveries of the past two decades — the solar systems out there are a menagerie of very different shapes and sizes, with exoplanets of a wild range of sizes orbiting an equally wide range of types and sizes of stars.

Michaël Gillon of the STAR Institute at the University of Liège in Belgium, and lead author of the discovery reported in the journal Nature, put it this way: “This is an amazing planetary system — not only because we have found so many planets, but because they are all surprisingly similar in size to the Earth.”

At a NASA press conference, he also said that “small stars like this are much more frequent than stars like ours.  Now we have seven Earth-sized planets to study, three in the habitable or ‘Goldilocks’ zone, and that’s quite promising for search for life beyond Earth.”

He said that the planets are so close to each other than if a person was on the surface of one, the others would provide a wonderful close-up view, rather like our view of the moon.

This diagram compares the orbits of the newly-discovered planets around the faint red star TRAPPIST-1 with the Galilean moons of Jupiter and the inner Solar System. All the planets found around TRAPPIST-1 orbit much closer to their star than Mercury is to the Sun, but as their star is far fainter, they are exposed to similar levels of irradiation as Venus, Earth and Mars in the Solar System.

The orbits of the Trappist-1 planets are not much greater than those of Jupiter’s Galilean moon system, and are considerably smaller than the orbit of Mercury in the solar system. However, TRAPPIST-1’s small size and low temperature mean that the energy reaching its planets is similar to that received by the inner planets in our solar system. TRAPPIST-1c, d and f, for instance, receive similar amounts of energy as Venus, Earth and Mars, respectively.

All seven planets discovered in the system could potentially have liquid water on their surfaces, the authors said, though their orbital distances make some of them more likely candidates than others.  So far there has been no confirmation of water on the planets, but the search has intensified.

“The energy output from dwarf stars like TRAPPIST-1 is much weaker than that of our sun,” co-author Amaury Triaud said.  “Planets would need to be in far closer orbits than we see in our solar system if there is to be surface water. Fortunately, it seems that this kind of compact configuration is just what we see around TRAPPIST-1.”

Climate models suggest the innermost planets, TRAPPIST-1b, c and d, are probably too hot to support liquid water, except maybe on a small fraction of their surfaces. The orbital distance of the system’s outermost planet, TRAPPIST-1h, is unconfirmed, though it is likely to be too distant and cold to harbor liquid water — assuming no alternative heating processes are occurring

TRAPPIST-1e, f, and g, however, represent the sweet spot for planet-hunting astronomers, as they orbit in the star’s habitable zone and could host oceans of surface water if other conditions were present.

As Sara Seager, an MIT pioneer in the study of exoplanet atmospheres, said in the NASA press conference, “with this discovery we’ve taken a giant, accelerated leap forward.  In one system, we have room so that if one planet in the habitable zones is not quite right (for study and possibly biology) we have many other chances.  This Goldilocks has many sisters.”

Thomas Zurbuchen, associate administrator of the agency’s Science Mission Directorate, added: “This discovery could be a significant piece in the puzzle of finding habitable environments, places that are conducive to life. Answering the question ‘are we alone’ is a top science priority and finding so many planets like these for the first time in the habitable zone is a remarkable step forward toward that goal.”

The telescope at the center of the discoveries is TRAPPIST-South (TRAnsiting Planets and PlanetesImals Small Telescope–South), s small 60 cm telescope at the La Silla Observatory in Chile devoted to the study of planetary systems.  The robotic telescope is operated from a control room in Liège, Belgium. The project is led by the Department of Astrophysics, Geophysics and Oceanography of the University of Liège, in close collaboration with the Geneva Observatory (Switzerland). TRAPPIST–South is mostly funded by the Belgian Fund for Scientific Research with the participation of the Swiss National Science Foundation. (ESO)

The discovery of the four additional Earth-sized planets was made during a global campaign of observataion, most especially by NASA’s infrared Spitzer Space Telescope.

Sean Carey, manager of NASA’s Spitzer Science Center at Caltech/IPAC in Pasadena, California, called it “the most exciting result I have seen in the 14 years of Spitzer operations…Spitzer will follow up in the fall to further refine our understanding of these planets so that the James Webb Space Telescope can follow up. More observations of the system are sure to reveal more secrets.”

Because the Trappist-1 system is so relatively easy to observe, and because it is providing such riches, many ground- based observatories have joined in the search.

Dips in the star’s light output caused by each of the seven planets passing in front of it — events known as transits — allowed the astronomers to infer information about their sizes, compositions and orbits. They found that at least the inner six planets are comparable in both size and temperature to the Earth.

These new discoveries make the TRAPPIST-1 system a very important target for future study. The Hubble Space Telescope is already being used to search for atmospheres around the planets and team leader Gillon said the James Webb Space Telescope, scheduled to launch in 2018, can potentially begin a rigorous examination of the atmospheres of the planets.

“These planets are accesible to observations with JWST.  We will be able to study the atmospheres, the greenhouse gas compositions.  We will search for gases that might be produced by life,” said Gillon.

This 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. The system has been revealed through observations from NASA’s Spitzer Space Telescope and the ground-based TRAPPIST (TRAnsiting Planets and and PlanetesImals Small Telescope) telescope, as well as other ground-based observatories.  (NASA)

But there are also significant barriers to habitability in the TRAPPIST-1 system.

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

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

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

Gillon said that the age of the star system was not well understood, but that it was at least a half billion years old.

Shawn Domogal-Goldman, a research space scientist at the Goddard Space Flight Center with a focus on exoplanets, said that the big news of the day for him is that the questions raised about conditions on red or M dwarf stars is that “they’re all testable on the TRAPPIST-1 planets in the near term.

“We can do follow-up observations of these worlds with the Hubble and JWST. Yesterday, I would have said ‘you can test these hypotheses with Webb but you kind of need the perfect target.’ Well, today we kind of have the perfect target.”

This diagram shows how the light of the dim red ultra cool dwarf star TRAPPIST-1 fades as each of its seven known planets passes in front of it and blocks some of its light. The larger planets create deeper dips and the more distance ones have longer lasting transits as they are orbiting more slowly. These data were obtained from observations made with the NASA Spitzer Space Telescope. (NASA)

From the total of 2,687 stars known to have exoplanets (as of February 15, 2017), there are a total of 602 known multiplanetary systems, or stars with at least two confirmed planets. About 280 of these have only two confirmed exoplanets, but some have a significantly larger number.

The star with the most confirmed planets is our sun with eight (after the demotion of Pluto), while the stars with the most confirmed exoplanets are Kepler-90, HD 10180 and HR 8832, with 7 confirmed planets each.  In 2012, two more exoplanet candidates were proposed but not yet confirmed for HD 10180, which would bring the total to 9 exoplanets in that system.

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Rocky, Close and Potentially Habitable Planets Around a Dwarf Star

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This artist’s impression shows an imagined view from the surface one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth that were discovered using the TRAPPIST telescope at ESO’s La Silla Observatory. (M. Kornmesser/ESO)
This artist’s impression shows an imagined view from the surface one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth that were discovered using the TRAPPIST telescope at ESO’s La Silla Observatory. (M. Kornmesser/ESO)

Forty light-years away is no small distance. But an announcement of the discovery of two planets at that separation that have been determined to be rocky and Earth-sized adds a significant new twist to the ever-growing collection of relatively close-by exoplanets that just might be habitable.

The two planets in the TRAPPIST-1 system orbit what is known as a red dwarf star, a type of star that is typically much cooler than the sun, emitting radiation in the infrared rather than the visible spectrum.  While there has been much debate about whether an exoplanet around a dwarf can be deemed habitable, especially since they are all believed to be tidally locked and so only one side faces the star, a consensus appears to be growing that dwarf stars could host habitable planets.

The two new rocky exoplanets were detected using the Hubble Space Telescope and were deemed most likely rocky by the compact sizes of their atmospheres — which were not large and diffuse hydrogen/helium envelopes (like that of the Jupiter) but instead more tightly packed, more like the atmospheres of Earth, Venus, and Mars.  It was the first time scientists have been able to search for and at least partially characterize of atmospheres around a temperate, Earth-sized planet.

Having determined that the planets are rocky, principal investigator Julien de Wit of M.I.T’s Department of Earth, Atmospheric and Planetary Sciences, said the goal now is to characterize their atmospheres.

“Now the question is, what kind of atmosphere do they have?” de Wit said. “The plausible scenarios include something like Venus, where the atmosphere is dominated by carbon dioxide, or an Earth-like atmosphere with heavy clouds, or even something like Mars with a depleted atmosphere. The next step is tomtry to disentangle all these possible scenarios that exist for these terrestrial planets.”

Artist's impression of the two planets in the Trappist-1 solar system. These worlds have sizes, temperatures and potentially atmospheres similar to those of Venus and Earth. Some believe they may be the best targets found so far for the search for life outside the solar system. They are the first planets ever discovered around such a tiny and dim star. (Nasa/ESA/STScI)
Artist’s impression of the two planets in the Trappist-1 solar system. These worlds have sizes, temperatures and potentially atmospheres similar to those of Venus and Earth. Some believe they may be the best targets found so far for the search for life outside the solar system. They are the first planets ever discovered around such a tiny and dim star. (Nasa/ESA/STScI)

 

Host stars with exoplanets that are (very relatively) close to us are highly valued because they are potentially easier to observe and characterize.

There are 24 known exoplanets within 40 light-years, 14 are within 30 light-years, and only six are within 20 light-years. The closest exoplanet considered confirmed by NASA is Epsilon Eridani b, 10.5 light-years away from our solar system, while the closest known rocky planet is HD 219134 b, which is 21 light-years away..  Planetary companions have been suggested to exist in some of the nine star systems located within 10 light-years away, including in the closest system, Alpha Centauri (4.1 light-years away).

TRAPPIST-1 (planets b and c) are among the closest orbiting a red dwarf star, and they provided an unusual double transit to observe.

“The two planets actually transited their star just 12 minutes apart so we got two planets for the price of one,” said co-author Hannah Wakeford of NASA’s Goddard Space Flight Center.  “This is the first time two planets have been characterized with Hubble at the same time on purpose, and the first time such small (Earth-sized) planets have had atmospheric follow-up done.”

The researchers hope to use Hubble to conduct follow-up observations to search for thinner atmospheres, composed of elements heavier than hydrogen, like those of Earth and Venus.

“With more data, we could perhaps detect methane or see water features in the atmospheres, which would give us estimates of the depth of the atmospheres,” she said.

The results were reported in the journal Nature.

Hubble/WFC3 white-light curve for the TRAPPIST-1b and TRAPPIST-1c double transit of 4 May 2016. (NASA/SScI)
Hubble/WFC3 white-light curve for the TRAPPIST-1b and
TRAPPIST-1c double transit of 4 May 2016. (NASA/STScI)

There’s an interesting story behind their Hubble observation of the two transits.  Using their relatively small telescope at the European Southern Observatory’s La Silla facility in Chile, the TRAPPIST-1 team detected the unusual three-planet system around the small, cool star and published their discovery a little more than two months ago.

Within days, they realized that planets b and c would be orbiting the star at almost exactly the same time — an unexpected and quite valuable occurrence.  (Information about that double transit was provided via the Spitzer Space Telescope, which had also been studying the orbits of planets in the TRAPPIST-1 system.)

The upcoming double transit was confirmed but two weeks before the event. The team requested Hubble time for a quick observation, and it was granted.  The successful observation soon followed.

DeWit said that planets with the sizes and equilibrium temperatures of TRAPPIST-1b and TRAPPIST-1c could possess relatively thick atmospheres with water, carbon dioxide, nitrogen and oxygen.

The TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) project is the creation in large part of the Origins in Cosmology and Astrophysics group of the University of Liege in Belgium.

The TRAPPIST instrument is new kind of ground telescope designed to survey the sky in infrared. TRAPPIST was built as a 60-centimeter prototype to monitor the 70 brightest dwarf stars in the southern sky. Now, the researchers have formed a consortium, called SPECULOOS (Search for habitable Planets Eclipsing ULtra-cOOl Stars), and are building four larger versions of the telescope in Chile, to focus on the brightest ultracool dwarf stars in the skies over the southern hemisphere. The researchers are also trying to raise money to build telescopes in the northern sky.

The 60cm telescope is devoted to the detection and characterization of planets located outside our Solar System and to the study of comets and other small bodies in our solar system. (Trappist/ESO)
The 60cm telescope is devoted to the detection and characterization of planets located outside our Solar System and to the study of comets and other small bodies in our solar system. (Trappist/ESO)

According to De Wit, he TRAPPIST telescopes are inexpensive compared with their peer — about $400,000 per instrument.

He is pushing to make them a relatively affordable “prescreening tool” that scientists can use to identify planets that are potentially habitable.  The TRAPPIST observations would then be followed up by my detailed study using powerful telescopes such as Hubble and NASA’s James Webb Telescope, which is scheduled to launch in October 2018.

“With more observations using Hubble, and further down the road with James Webb, we can know not only what kind of atmosphere planets like TRAPPIST-1 have, but also what is within these atmospheres,” de Wit says. “And that’s very exciting.”

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A Dwarf Star Produces a Major Discovery

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 

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