What TRAPPIST-1's freezing outermost planet could tell us

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May 31, 2017

Exoplanet TRAPPIST-1h is basically a space fridge with an average temperature of negative 148 Fahrenheit—but it has a lot more to tell us than how much frozen pizza is left.

The outermost planet in the TRAPPIST-1 star system (whose discovery made the internet explode not too long ago) is predictably the coldest. TRAPPIST-1 is an ultra-cool dwarf star whose luminosity is nowhere near that of our sun. That doesn’t exactly mean tropical weather for the planet orbiting furthest from it. Ironically, scientists believe that when its host star blazed brighter, TRAPPIST-1h may have been warm and even habitable for several hundred million years, before freezing over.

TRAPPIST-1h was destined for eternal winter from the moment it fell into orbit. When an exoplanet is in transit of its host star, it passes directly in front of it, blocking starlight and making itself visible to an observer. The team behind the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) survey, led by astronomer Michael Gillion, only detected a single transit for the exoplanet until doctoral student Rodrigo Luger’s team was able to later confirm and study four transits using extensive data from the Kepler Space Telescope’s second mission, K2. His study was recently published in the journal Nature Astronomy.

K2 also shed light on TRAPPIST-1h’s rotation and activity by characterizing the orbits of its six siblings, so they could be recognized and ruled out in order to study the seventh planet. What the team didn’t expect to discover was an orbital resonance, meaning all seven planets’ orbital periods are interconnected mathematically, and are therefore able to influence each other. Orbital resonance is what allowed the team to use the other planets’ orbital velocities to predict that of TRAPPIST-1h. Backchecking against the K2 data proved this method was accurate.

Orbital resonance enables this type of research because of its unshakable stability. If even one planet is slightly knocked off course in some sort of celestial roller derby, it would bounce right back into the resonance. Escape is virtually impossible. Orbital resonance is also an indicator that the connections were forged during the early days of planetary and orbital formation within the TRAPPIST-1 system, barely after the planets had time to cool off. This is the first time as many as seven planets have been known to be in resonance. Ever.

"The resonant structure is no coincidence, and points to an interesting dynamical history in which the planets likely migrated inward in lock-step," said Luger. "This makes the system a great testbed for planet formation and migration theories."

Luger is currently studying the K2 data for signs of any spacecraft glitch that could have blotted out planetary signals, and is optimistic that this star system’s prime location will make the study of their atmospheric composition a possibility in the near future.

(via Phys.org)