TRAPPIST-1 is Older Than Our Solar System

If we want to know more about whether life could survive on a
planet outside our solar system, it’s important to know the age
of its star. Young stars have frequent releases of high-energy
radiation called flares that can zap their planets’ surfaces.
If the planets are newly formed, their orbits may also be
unstable. On the other hand, planets orbiting older stars have
survived the spate of youthful flares, but have also been
exposed to the ravages of stellar radiation for a longer period
of time.

Scientists now have a good estimate for the age of one of the
most intriguing planetary systems discovered to date —
TRAPPIST-1, a system of seven Earth-size worlds orbiting an
ultra-cool dwarf star about 40 light-years away. Researchers
say in a new study that the TRAPPIST-1 star is quite old:
between 5.4 and 9.8 billion years. This is up to twice as old
as our own solar system, which formed some 4.5 billion years
ago.

The seven wonders of TRAPPIST-1 were revealed earlier this year
in a NASA news conference, using a combination of results from
the Transiting Planets and Planetesimals Small Telescope
(TRAPPIST) in Chile, NASA’s Spitzer Space Telescope, and other
ground-based telescopes. Three of the TRAPPIST-1 planets reside
in the star’s “habitable zone,” the orbital distance where a
rocky planet with an atmosphere could have liquid water on its
surface. All seven planets are likely tidally locked to their
star, each with a perpetual dayside and nightside.

At the time of its discovery, scientists believed the
TRAPPIST-1 system had to be at least 500 million years old,
since it takes stars of TRAPPIST-1’s low mass (roughly 8
percent that of the Sun) roughly that long to contract to its
minimum size, just a bit larger than the planet Jupiter.
However, even this lower age limit was uncertain; in theory,
the star could be almost as old as the universe itself. Are the
orbits of this compact system of planets stable? Might life
have enough time to evolve on any of these worlds?

“Our results really help constrain the evolution of the
TRAPPIST-1 system, because the system has to have persisted for
billions of years. This means the planets had to evolve
together, otherwise the system would have fallen apart long
ago,” said Adam Burgasser, an astronomer at the University of
California, San Diego, and the paper’s first author. Burgasser
teamed up with Eric Mamajek, deputy program scientist for
NASA’s Exoplanet Exploration Program based at NASA’s Jet
Propulsion Laboratory, Pasadena, California, to calculate
TRAPPIST-1’s age. Their results will be published in The
Astrophysical Journal.

It is unclear what this older age means for the planets’
habitability. On the one hand, older stars flare less than
younger stars, and Burgasser and Mamajek confirmed that
TRAPPIST-1 is relatively quiet compared to other ultra-cool
dwarf stars. On the other hand, since the planets are so close
to the star, they have soaked up billions of years of
high-energy radiation, which could have boiled off atmospheres
and large amounts of water. In fact, the equivalent of an Earth
ocean may have evaporated from each TRAPPIST-1 planet except
for the two most distant from the host star: planets g and h.
In our own solar system, Mars is an example of a planet that
likely had liquid water on its surface in the past, but lost
most of its water and atmosphere to the Sun’s high-energy
radiation over billions of years.

However, old age does not necessarily mean that a planet’s
atmosphere has been eroded. Given that the TRAPPIST-1 planets
have lower densities than Earth, it is possible that large
reservoirs of volatile molecules such as water could produce
thick atmospheres that would shield the planetary surfaces from
harmful radiation. A thick atmosphere could also help
redistribute heat to the dark sides of these tidally locked
planets, increasing habitable real estate. But this could also
backfire in a “runaway greenhouse” process, in which the
atmosphere becomes so thick the planet surface overheats – as
on Venus.

“If there is life on these planets, I would speculate that it
has to be hardy life, because it has to be able to survive some
potentially dire scenarios for billions of years,” Burgasser
said.

Fortunately, low-mass stars like TRAPPIST-1 have temperatures
and brightnesses that remain relatively constant over trillions
of years, punctuated by occasional magnetic flaring events. The
lifetimes of tiny stars like TRAPPIST-1 are predicted to be
much, much longer than the 13.7 billion-year age of the
universe (the Sun, by comparison, has an expected lifetime of
about 10 billion years).

“Stars much more massive than the Sun consume their fuel
quickly, brightening over millions of years and exploding as
supernovae,” Mamajek said. “But TRAPPIST-1 is like a
slow-burning candle that will shine for about 900 times longer
than the current age of the universe.”

Some of the clues Burgasser and Mamajek used to measure the age
of TRAPPIST-1 included how fast the star is moving in its orbit
around the Milky Way (speedier stars tend to be older), its
atmosphere’s chemical composition, and how many flares
TRAPPIST-1 had during observational periods. These variables
all pointed to a star that is substantially older than our Sun.

Future observations with NASA’s Hubble Space Telescope and
upcoming James Webb Space Telescope may reveal whether these
planets have atmospheres, and whether such atmospheres are like
Earth’s.

“These new results provide useful context for future
observations of the TRAPPIST-1 planets, which could give us
great insight into how planetary atmospheres form and evolve,
and persist or not,” said Tiffany Kataria, exoplanet scientist
at JPL, who was not involved in the study.

Future observations with Spitzer could help scientists sharpen
their estimates of the TRAPPIST-1 planets’ densities, which
would inform their understanding of their compositions.

For more information about TRAPPIST-1, visit:

https://exoplanets.nasa.gov/trappist1

News Media Contact

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov

2017-214

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