The TRAPPIST-1 Planets Are Aligned With Their Star's Spin, Removing One Obstacle To Life

An artist's impression of the low-mass TRAPPIST-1 star and some of its seven planets. We now know the stellar equator is roughly aligned with the plane of these orbits. NAOJ

Astronomers have found that the orbits of the seven-planet TRAPPIST-1 system are approximately aligned with the equator of their parent star. The finding, besides requiring an astonishing feat of measurement, removes one possible obstacle to the system’s habitable zone being inhabited, although larger ones remain. 

The solar system’s planets have orbits that lie almost in a plane, which is within 6 degrees of the solar equator. This is consistent with our ideas about star formation – the Sun and planets formed out of a rotating disk and each maintains the angular momentum from its spin.

Consequently, astronomers expected to see something similar when it became possible to detect planets around other stars as well as the axis of these stars' spin. Yet where both have been measured, some of the stars are misaligned, not just by small amounts that can be waived away but with planets passing almost over the stellar poles. This is rare in multi-planet systems but not unknown. Either these systems’ planetary orbits and stellar rotations were never aligned in the first place or some event threw them off.

Astronomers are uncertain which of these seemingly improbable options is less likely. The answer potentially matters for the quest to find life beyond the Solar System. Anything that alters the orbits of planets so drastically, or causes the star to keel over on its side, is unlikely to be good for the life that has established itself in the system.

This brings us to the TRAPPIST-1 system, where three of the seven planets lie in the “habitable zone”, a region where temperatures are likely to be suitable for liquid water. TRAPPIST-1 is among the best places to look for life, not only because there are three prospects in the one system but because it’s one of the closest stars with known rocky planets. Moreover, the orbital plane of the planets aligns with Earth, so they pass between us and the star. That's how we discovered them in the first place, and it also makes them much easier to study than planets discovered by other means.

However, we've never managed to measure the alignment between planets orbiting a low-mass star like TRAPPIST-1 and the stellar rotation. Dr Teruyuki Hirano of the Tokyo Institute of Technology and his team have changed that. Hirano used the Subaru Telescope to observe TRAPPIST-1 on a night when three of its planets, including two in the habitable zone, passed across its face. In The Astrophysical Journal Letters, Hirano reports an apparent misalignment of just 1 degree – even less than in our own system. 

“The precision of the measurements was not good enough to completely rule out a small spin-orbit misalignment,” Hirano acknowledged in a statement. "Nonetheless, this is the first detection of the effect with Earth-like planets and more work will better characterize this remarkable exoplanet system."

The paper calls for further measurements, but its findings reduce the chances TRAPPIST-1 has suffered some catastrophic event.

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