The world has been looking at TRAPPIST-1, the cool M dwarf with 7 potentially habitable rocky exoplanets. As part of the ardent interest in this diminutive star, Vincent Bourrier and colleagues are trying to find how much radiation is “thrown” at the planets from their own sun.
The researchers used the Hubble telescope and looked at the star’s ultraviolet output. They were looking for Lyman alpha emissions, wavelength emitted by hydrogen atoms from the chromosphere – the layer beneath the star’s surface (a.k.a photosphere)
The team found out that TRAPPIST-1 emitted half of the Lyman emissions as other cool M dwarfs.
Another finding of the team last year was that it emitted as much X-rays as Proxima Centauri.
The X-ray and ultraviolet output of these type of stars decays over time, but the X-rays drop much faster. The scientists posits that TRAPPIST-1 is an young star, because it emits only 1/3 Lyman alpha emissions compared to its X-rays.
But exactly how old is the star is an open question. The astronomers believe that it is at least 500 millions old.
Jeffrey Linsky study
Jeffrey Linsky, astronomer from University of Colorado, who was studied for a long time M dwarfs’ Lyman alpha emissions and X-rays, says that the star looks both old and young When the stars are born they spin quickly. As they age, they become slower. TRAPPIST-1 spins every 1,5 days which points out that it is a young star. However, astronomers don’t know how fast ultracool dwarfs spin down.
Bourrier says that the question about its age is currently unanswerable. The difference between X-rays and Lyman alpha emissions suggest that the star is not extremely old, he says, “but I do not think that at this point we can say much more than this.”
The reason why X-rays and ultraviolet levels matter its because they can harm habitability. While the ultraviolet level is low it can strip the atmospheres of b and c planets in 1-3 billions. For the next four d, e, f, g the process can take between 5 to 22 billion years.
Because of the world’s methodical spacing, they most likely migrated inward. Currently we have no idea when this happened or if their orbits are going to be stable for long. “If they migrated within a disk, typical time scales are about 100 million years, but that may not be valid for a system like TRAPPIST-1,” Bourrier warns. “Uncharted territory here!”