5Discoveries of planets around distant stars have become almost routine. But finding seven exoplan- ets in one go is something special. In February, a team of planet seekers announced that a small, cool star some 39 light-years away, TRAPPIST- 1, hosts the most Earth-sized exoplanets yet found
in one place: seven roughly Earth-sized worlds, at least three
of which might host liquid water (SN: 3/18/17, p. 6).
These worlds instantly became top priorities in the search
for life outside the solar system. “TRAPPIST- 1 is on everybody’s wish list,” says exoplanet astronomer Lisa Kaltenegger
of Cornell University. But the planets and their dim star have
also stoked a raging debate about what makes a planet habitable in the first place.
Astrophysicist Michaël Gillon of the University of Liège in
Belgium and colleagues found the family of worlds orbiting the
ultracool dwarf star, dubbed TRAPPIST- 1 for the small telescope in Chile used to discover its planets.
“I don’t think the cachet of that system is going away anytime soon,” says exoplanet expert Sara Seager of MI T.
The TRAPPIST telescope team first announced in May 2016
that the star had three temperate, rocky planets. Staring at
the system with the Spitzer Space Telescope for almost three
weeks straight revealed that the third planet was actually four
more — all Earth-sized, and three of them are in the star’s habitable zone, the region where temperatures are right for liquid
water on a planet’s surface. A seventh planet was caught crossing the star as well, though follow-up observations showed it is
too cold for life as we know it (SN: 6/24/17, p. 18).
The number of worlds alone makes the TRAPPIS T- 1 system
a good spot to look for life. An alien observing our solar system
would think Venus, Earth and Mars all fall in the habitable zone.
But only one is inhabited. The fact that TRAPPIST- 1 has so many
options increases the odds that the system hosts life, Seager says.
Seven Earth-sized planets
orbit the same ultracool star
By Lisa Grossman
As an ultracool dwarf, TRAPPIST- 1 rides the edge of what
counts as a star. Such stars burn through their nuclear fuel so
slowly that they can live for many billions of years, which gives
any life on their planets a long time to grow and evolve. This
star’s habitable zone is also incredibly close in, offering astronomers many chances to observe the planets orbiting their star.
The three planets in the habitable zone cross in front of the
star every 6. 10, 9. 21 and 12. 35 days. If two or more turn out to
be habitable, then they could share life among them, either by
tossing meteorites back and forth or — in the case of spacefar-ing civilizations — by deliberate space travel.
Future space-based observatories will be able to see starlight filtering through the planets’ atmospheres, if the planets
have atmospheres. Gillon and colleagues are looking for signs
of escaping hydrogen, a signal that an atmosphere might be
there. “We’re already preparing,” he says.
But ultracool dwarfs are also ill-tempered. They tend to emit
frequent, powerful stellar flares, which could rip away a planet’s
atmosphere, threatening any potential for life. The planet-hunting Kepler space telescope recently watched TRAPPIST- 1
for 80 days and saw it flare 42 times. One of those flares was as
strong as Earth’s 1859 Carrington Event, among the strongest
geomagnetic storms ever observed.
But there are other promising systems. Recently, a similar
star, Ross 128, only 11 light-years from Earth and much calmer
than TRAPPIST- 1, was found to have an Earth-mass planet,
making it a better place to look for life, researchers reported
in November in Astronomy & Astrophysics. Whether such stars
are good or bad for life is an old and open question (SN: 6/24/17,
p. 18). TRAPPIST- 1’s advantage is in its numbers. “We can check
it, not just with one planet but with many planets,” Kaltenegger
says. “You have hotter than Earth, like Earth and colder than
Earth. If you wanted Goldilocks, this is the ideal scenario.”
TRAPPIST- 1 is just an opening act. A bigger, more sensitive observatory called SPECULOOS is expected to be fully
operational in the Chilean desert in early 2019, Gillon says.
SPECULOOS will seek planets around 1,000 ultracool dwarf
stars over 10 years. “We are at the edge of maybe detecting life
around another star,” he says. “It’s really a possibility.” s
The small, cool star TRAPPIST- 1,
illustrated, hosts a bevy of Earth-sized
planets. There could be many more
stars like it worth studying.