The intriguing ultra-cool dwarf star of the TRAPPIST-1 system of seven Earth-size worlds could be up to twice as old as our own solar system, says a new study.
TRAPPIST-1 star is between 5.4 and 9.8 billion years, while our own solar system formed some 4.5 billion years ago, according to the results to be published in The Astrophysical Journal.
If we want to know more about whether life could survive on a planet outside our solar system, it is 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.
"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.
The seven wonders of TRAPPIST-1 were revealed earlier this year in 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 that long to contract to its minimum size.
However, even this lower age limit was uncertain. In theory, the star could be almost as old as the universe itself.
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.
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.
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.
"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.
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