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.
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 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.
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.”
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.
Many Worlds 