The search for exoplanets – planets orbiting other stars – has been one of the most exciting developments in astronomy and space science in recent years. The first couple exoplanets were found in 1992, and now over 3,400 have been confirmed, with over 5,000 additional candidates. Some of these are smaller rocky worlds similar in size to Earth, bringing scientists close to finding “Earth 2.0” – another planet with water and, perhaps, life. Today, NASA announced another key discovery, bringing us even closer to finding another living world – a star with not just one or two Earth-sized planets orbiting it, but seven. Three of those planets are in the star’s habitable zone, where, depending upon other surface conditions, lakes or oceans of liquid water could exist.
Actually, all of the planets are considered to be temperate, and any of them could possibly have water depending on surface conditions, but the three in the habitable zone are the most likely. This is also a new record for the number of habitable zone planets, and Earth-sized planets, orbiting a single star.
“This discovery could be a significant piece in the puzzle of finding habitable environments, places that are conducive to life,” said Thomas Zurbuchen, associate administrator of the agency’s Science Mission Directorate in Washington. “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 seven planets orbit a star known as TRAPPIST-1, which is about 40 light-years (235 trillion miles) away, in the constellation Aquarius.
Three of the planets had been known about previously, initially found by astronomers using The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile in May 2016. Now, NASA’s Spitzer Space Telescope, along with the European Southern Observatory’s Very Large Telescope, has confirmed two of those planets again as well as five more.
As also noted by lead author Michaël Gillon of the STAR Institute at the University of Liège in Belgium, “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!”
The new results were published today in the journal Nature.
Using Spitzer, astronomers were able to measure the sizes of all seven planets, as well as an estimate of mass for six of them, which also allows their density to be calculated. The density measurements show that all seven planets are rocky, like Earth. The known masses for six of the planets range from 0.41 to 1.38 times that of the Earth. It is thought that the seventh and farthest planet might be an “icy, snowball-like” world, but more observations are needed to determine whether this is accurate or not. The same applies to the other six planets as well; determining what the actual conditions are like on any of them will require much more observation. Atmospheric conditions and other factors can widely affect the potential habitability of a planet, even if it is the habitable zone of a star.
“The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star,” said Gillon. “It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds.”
TRAPPIST-1 itself is a cool dwarf star, much smaller than our Sun, and only slightly larger than Jupiter. Because of this, the energy output is weaker, but that doesn’t mean at least some of the planets could be potentially habitable. All seven of the planets orbit very close to the star, closer than Mercury to our own Sun. But since the energy output is less, that means the habitable zone is also closer in to the star as compared to our Solar System, and three of those planets are right inside it.
As noted in the ESO press release, “The energy output from dwarf stars like TRAPPIST-1 is much weaker than that of our Sun. Planets would need to be in far closer orbits than we see in the 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!”
All seven planets may also be tidally locked, meaning that the same side of the planet is always facing the star, much like how the same side of the Moon always faces the Earth. If so, there could be strong effects on weather systems, such as strong winds blowing from the day side to the night side or extreme temperature changes.
The seven planets are also quite close to each other, and would appear about the same size or larger than our Moon in each other’s skies. It’s the kind of thing that science fiction stories are made of.
The new findings show again how diverse planetary systems can be. They will also provide clues to how smaller planets form.
“This system is going to be one of the best laboratories we have for understanding the evolution of small planets,” said Zachory Berta-Thompson, an astronomer at the University of Colorado Boulder.
“This is the most exciting result I have seen in the 14 years of Spitzer operations,” said Sean Carey, manager of NASA’s Spitzer Science Center at Caltech/IPAC in Pasadena, California. “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.”
Future observations, such as with the upcoming James Webb Space Telescope, will try to determine how many of these planets have atmospheres, and what the compositions are. With these or other exoplanets, astronomers are hoping to find other worlds with atmospheres similar to Earth’s, including the possible detection of biosignatures – gasses which could indicate biological activity such as oxygen or methane. As far as is currently known, based on observations by the Hubble Space Telescope, the two innermost planets, at least, do not have “puffed-up” atmospheres like those of gas or ice giants, meaning that they any atmospheres they have would be more like those of terrestrial planets such as Venus, Earth or Mars.
“The TRAPPIST-1 system provides one of the best opportunities in the next decade to study the atmospheres around Earth-size planets,” said Nikole Lewis, co-leader of the Hubble study and astronomer at the Space Telescope Science Institute in Baltimore.
Earth-sized planets have been found orbiting other stars as well, including the nearest star Proxima Centauri, which has at least one Earth-mass world, Proxima b. It is only slightly more massive than Earth and also orbits in that star’s habitable zone. Proxima Centauri is a red dwarf star, also smaller and cooler than the Sun. Having seven such worlds in one system makes TRAPPIST-1 rather unique however. The first Earth-mass planet orbiting a red dwarf star was discovered in 2014, called OGLE-2013-BLG-0341LBb, which has a mass no more than twice that of Earth and orbits at a very similar distance from its star that Earth does from the Sun.
Scientists now estimate that there are billions of Earth-sized worlds in our galaxy alone. Studying the TRAPPIST-1 planets, which are relatively close, will help scientists learn more about what other similar planets might be like, and will hopefully bring us closer to finding the ultimate treasure – another inhabited world.
The SETI Institute, which uses radio telescopes to search for intelligent life via radio or optical signals, turned its Allen Telescope Array to the TRAPPIST-1 system last year, but didn’t find anything, as mentioned in a CBC News article by Nicole Mortillaro. Disappointing, but that was only one search so far. As Seth Shostak, a senior astronomer at SETI put it:
“It shows that there’s just an awful lot of territory on which you could have life. When you have tens of billions of habitable worlds – moons and planets – just in our galaxy, that’s a stunningly large number of worlds where there could be life. It takes a daring person to say, ‘We’re the only one where anything interesting is happening.’ That’s a bit self-centred, I’d say.”
Whether any of these Earth-sized worlds are home to alien biology, however simple or complex, is simply unknown at this point. As for TRAPPIST-1 though, as noted by Gillon, “The story is just beginning.”
And perhaps someday, we will be able to send probes there, to actually see these wondrous worlds up close.
More information about the Spitzer Space Telescope is available here.