TRAPPIST-1: Come On, How Many Planets Are Enough?

Remember TRAPPIST-1? That ultra-cool dwarf star with a miniaturized solar system of seven planets?

Whenever we talk about TRAPPIST-1, we really should specify that it has seven planets that we know about. Astronomers are still searching for more. Specifically, they’re searching for larger, Jupiter-like bodies.

So far, astrometric observations (precise measurements of a star’s gravitational “wobble”) have ruled out some possibilities. There are no planets in the TRAPPIST-1 System with masses greater than 4.6 times the mass of Jupiter with orbital periods of one year or less, and no planets with masses greater than 1.6 times the mass of Jupiter with orbital periods of five years or less.

That still leaves the door open for a lot of other very large planets. Just imagine if we discover a couple of Saturn-mass objects, or half a dozen Neptunes. Heck, there could still be Jupiter-mass planets out there! Or maybe not. It could just be the seven Earth-sized planets we already know about.

As explained in this article from Centauri Dreams, there are currently two competing theories to explain how gas giants form. One of these theories would probably allow gas giants to form around TRAPPIST-1; the other probably would not.

  • Core Accretion: a large, rocky core forms first and then envelops itself in gases from the proto-planetary disk surrounding a newborn star. This would be a very slow formation process.
  • Disk Instability: The proto-planetary disk surrounding a young star “destabilizes,” forming whispy structures like the spiral arms of a galaxy. Knots of gas in these spiral arms would condense into planet shapes, and the rocky cores of these planets would form later (or in some cases perhaps not at all) from asteroids or other debris captured by the gas giant’s gravity. This process would happen quickly.

Given what we know so far about TRAPPIST-1, it’s unlikely gas giants could have formed there by core accretion. TRAPPIST-1’s protoplanetary disk wouldn’t have been around long enough. Therefore if we find gas giants orbiting TRAPPIST-1, that would challenge the core accretion model and give more credence to disk instability.

So now the search is on!

Will we find gas giants around TRAPPIST-1? I kind of hope we do. First off, it would make TRAPPIST-1 even more awesome than it already is. And secondly, it would mean the core accretion model—the traditionally accepted view among astrophysicists—is wrong, or at least incomplete, and when theories turn out to be wrong or incomplete, that’s when the real fun of science begins.

2 thoughts on “TRAPPIST-1: Come On, How Many Planets Are Enough?

  1. Okay Mars is too close so that got boiled and ended up accreted, melted with a scarred landscape. Earth got lucky with our distance from the sun and the moon which helps with the occasional asteroid not that most of them fly into the sun anyway. So if scientists manage to locate a magic three combination, Earth, moon and sun in the Trappist system we’ve likely discovered a new civilization with lifeforms! Exciting stuff and we still haven’t built a starship yet.


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