The TRAPPIST-1 planetary system, described Wednesday in the journal Nature, marks the first time so many terrestrial planets have been found around a single star.
Although scientists believe the planets are rocky and Earth-sized, too little is known about their atmospheres and other factors to say whether they are truly Earth-like.
“They could have some liquid water and maybe life, by extension, on the surface — especially three of these planets that are in the so-called habitable zone on the star,” said study leader Michael Gillon, a researcher at the Universite de Liege in Belgium.
The TRAPPIST-1 star is an ultracool dwarf star. It’s about 80 times more massive than the gas giant Jupiter and roughly 200 times fainter than the sun.
Even so, ultracool dwarf stars can be pretty hot places to look for potentially life-friendly planets.
In this case, that’s partly because all seven worlds orbit so close to the star’s surface, closer than Mercury is to the sun. With that kind of proximity, even the dwarf star’s dim light may provide enough warmth to support living things.
On top of that, the planets’ tight orbits make them very easy for certain telescopes to find.
The European Southern Observatory’s TRAnsiting Planets and PlanetesImals Small Telescope (TRAPPIST for short) in Chile uses the transit method to hunt for planets. As a planet passes, or transits, in front of its host star, it blots out a little bit of starlight, causing a dip in overall brightness that scientists can measure. If such a dip happens once, it could be a fluke. If it happens three or more times at regular intervals, it’s probably an orbiting planet.
If there are multiple planets, scientists can find them by looking at how they distort each other’s orbits. If a planet seems to transit a tad too early or too late, for example, it means that something else besides the star — such a fellow planet — is tugging on it. This information also allows astronomers to make a rough calculation of the other planet’s mass.
Astronomers announced the discovery of three planets around TRAPPIST-1 last year, but even then they suspected there might be a few more.
So they observed the star for 20 days with NASA’s Spitzer Space Telescope, which is managed by the Jet Propulsion Laboratory in La Canada Flintridge, Calif. The space telescope captured 34 transits of seven different planets. (Because the outermost planet only passed by the star only once, the scientists could not determine its exact orbit.)
All seven planets in the TRAPPIST-1 system are probably rocky, with masses in a range of 20 percent more to 20 percent less than that of Earth, give or take.
The dwarf star and exoplanets actually resemble an oversized Jupiter with its many moons, Gillon and his colleagues said. Like the Jovian satellites, TRAPPIST-1’s planets are in such tight orbits that they are probably tidally locked. If so, that means they show the same face to the star at all times, rather like the moon does to the Earth.
The seven planets also seem to be orbiting in resonance with each other. These gravitational interactions could mean that the planets are being heated by tidal forces.
Whether that’s good or bad depends on what kind of world you are. For Saturn’s icy moon Enceladus, a little tidal heating goes a long way, powering polar geysers more powerful than all the hot springs in Yellowstone. For Jupiter’s moon Io, tidal forces caused it to become covered in inhospitable-looking volcanoes.
“In the past few years, evidence has been mounting that Earth-sized planets are abundant in the Galaxy, but Gillon and collaborators’ findings indicate that these planets are even more common than previously thought,” astronomer Ignas Snellen of Leiden Observatory in the Netherlands wrote in a commentary that accompanied the paper.
Of course, whether these planets have the right conditions and chemical ingredients for life remains a question for future study. Observatories like NASA’s James Webb Space Telescope, set to launch in 2018, could analyze these planets’ atmospheres, perhaps providing important clues.
“Could any of the planets harbour life? We simply do not know,” Snellen wrote. “But one thing is certain: in a few billion years, when the Sun has run out of fuel and the Solar System has ceased to exist, TRAPPIST-1 will still be only an infant star. It burns hydrogen so slowly that it will live for another 10 trillion years … which is arguably enough time for life to evolve.”
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