TRAPPIST-1: When Icy Planets Thaw

Last week we talked about TRAPPIST-1 and its seven planets.  Turns out those planets have a whole lot of water (or at least they have very low densities, so they probably have a whole lot of water).  And yes, it’s entirely possible that something could be swimming around in all that water.  But the paper I cited last week wasn’t really about water or alien life.  Not really.

I mean, the stuff about water was important, but it wasn’t the real point of the paper.  The real point was that such water-rich planets could not have formed so close to their star.  They must have formed farther away, somewhere beyond TRAPPIST-1’s frost line, so that they’d be able to accumulate large quantities of water (and/or other volatiles) in the form of ice.  Then they migrated inward.

It would be sort of like if the ice-covered world of Pluto, or any of the large, icy moons of the Outer Solar System, were suddenly transplanted to the Inner Solar System.  All that frozen nitrogen and frozen methane would sublimate, turning into a generously thick atmosphere.  And all that frozen water would melt, turning into a deep, deep ocean—a global ocean so deep it would make Earth’s oceans look like puddles.

That’s what the TRAPPIST-1 planets are probably like: Pluto-like worlds that thawed.

The inward migration of the TRAPPIST-1 planets—sorry, I mean exoplanets—is sort of the opposite of what happened in our own Solar System.  Our gas giants, according to the Nice model, started out closer to the Sun and then migrated away (except for Jupiter, which moved a little closer to the Sun).

That was the real point of that paper I cited last week. This is also the kind of thing that made TRAPPIST-1 so scientifically interesting in the first place: the alignment of those seven exoplanets makes it really easy for us to study orbital dynamics in a multi-planetary system, and to compare and contrast what we learn with what we know about our own Solar System.

The stuff about water and potential alien life… that was just a nice bonus.

Sciency Words: Degeneracies

Today’s post is part of a special series here on Planet Pailly called Sciency Words.  Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


Okay, I have to be honest with you: I really don’t understand what this term means.  It’s a statistics thing, and it gets really mathy.  But since I came across this term in a paper about the TRAPPIST-1 planets, I felt I should try to get some sense of what a degeneracy is.  What I learned, at least in relation to planets, was interesting enough that I thought it was worth sharing with you.

Imagine we’re playing a game of “Guess Who?”  You know my person has red hair, but you still don’t know my person’s age or gender, you don’t know if my person is wearing glasses, or if my person has freckles.  That one datapoint—my person has red hair—eliminates a lot of possibilities from the board, but there are still plenty of possibilities left over.

Those left over possibilities can be refered to as degeneracies (if I’m understanding the proper usage of this term).  In that paper on the TRAPPIST-1 planets, it says: “The derivation of a planetary composition from only its mass and radius is a notoriously difficult exercise because of the many degeneracies that exist.”

In other words, if you’re playing “Guess Who?” with planets, knowing a planet’s mass and volume (and thus being able to calculate its density) still leaves you with a whole lot you don’t know about that planet.

This reminds me a lot of the Earth Similarity Index and the problems with using that system to identify Earth-like planets. Venus, for example, scores rather highly on the E.S.I. because its mass and volume are so similar to Earth’s, but Venus is not at all Earth-like in the sense that most people mean when they talk about Earth-like planets.

I’d say I plan to study this concept more, but I think I’m done for now.  I tried to read this paper from 2010 which seems to have introduced the subject of degeneracies to planetary science and warned that they’d be a real problem in the study of exoplanets.  But after attempting to slog my way through that paper, I think I’ve had enough mathy stuff for a while.

TRAPPIST-1: Too Much Water to Support Life?

I’m still catching up on my research after having something of a rough start to the year.  A few months ago, I saw headlines saying that water had been discovered in the TRAPPIST-1 system.  A whole lot of water.  Too much water, in fact.  Normally where there’s water there could be life, but according to the news articles I read back in February, the TRAPPIST-1 planets have so much water that life probably could not exist there (not enough carbon, not enough minerals).


But now I’ve finally read the actual research, and I’m really glad I did because a lot of journalists in the popular press clearly did not.  This paper, titled “Inward Migration of the TRAPPIST-1 Planets as Inferred From Their Water-Rich Compositions,” ends thusly:

[…] while these planets may be habitable in the classical definition, any biosignature observed from these planets system may not be fully distinguishable from abiotic, purely geochemical sources.  Thus, while M-dwarfs may be the most common habitable planet-host in our Galaxy, they may be the toughest on which to detect life.

In other words, these planets very well might be able to support life, but we may not be able to detect that life if it’s there.

This reminds me of a paper Carl Sagan wrote in the 1990’s showing how difficult it is to conclusively prove there is life on Earth based solely on observations made by a passing NASA spacecraft.

Earth’s oceans in particular do an outstanding job masking the usual biosignatures we would be looking for.  As far as that NASA spacecraft could tell, Earth’s oceans appeared to be completely lifeless.

So if the planets of the TRAPPIST-1 system really are as that inward migration paper describes them—15% to 50% water, with their surfaces covered entirely by ocean—then we are going to have a really difficult time finding life there.  But that is not the same as saying there’s no life there to find!

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.

TRAPPIST-1: A Sky Full of Planets

Okay, one more post about TRAPPIST-1 and its seven planets, and then I promise I’ll move on to another topic. But this is something that’s just too awesome for me to skip.

You know that goofy trope you sometimes see in Sci-Fi movies or comic books? The one where the hero is standing on the surface of some alien planet and there are a whole bunch of other planets in the sky? Like, not just a moon or two, but a ton of huge planets looming over the horizon.

Well, apparently if you stood on the surface of one of the planets in the TRAPPIST-1 system, you’d be able to look up and see the other planets in the sky. Not just as tiny points of light but as large orbs.

The planets of TRAPPIST-1 are packed extremely close together, it seems. Several articles I’ve read, such as this one from Spaceflight 101, suggest that weather patterns and surface features would be visible to the naked eye.

I’m guessing the view would not be quite as epic as what I drew for the illustration above, but still… it would be stunning to see it. Just remember to bring proper radiation gear. TRAPPIST-1 is still a flare star.

Sciency Words: Ultra-Cool Dwarf Star

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Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


At some point, I want to profile each of the planets in the TRAPPIST-1 system one by one for my Exoplanet Explorer series. But it’s too early for that. Right now, we don’t know much about these planets except that they’re there.

But I can say something about TRAPPIST-1 itself. It’s a type of star called an ultra-cool dwarf star.


Apparently TRAPPIST-1 has just barely enough mass to cause hydrogen fusion in its core. That means that for a star, it doesn’t produce a whole lot of energy, and thus its temperature is relatively low. Based on my rough math and statistics I got from Wikipedia, it looks like TRAPPIST-1 is less than half the temperature of our Sun.

This is one of the things that makes TRAPPIST-1 so interesting to me, and why it’s really starting to capture my imagination. It’s not just about all those Earth-like planets. The star itself helps set a lower limit for just how small and cold stars can be.

TRAPPIST-1: A Mini-Solar System

Right now, TRAPPIST-1 is getting a ton of attention. If feels like just about every single telescope on Earth or in Earth-orbit has been stealing glances of this very tiny star and its seven Earth-like planets.


But we’ve discovered lots of other exoplanets, many of them Earth-like, and many of them in multi-planet systems. So why is TRAPPIST-1 getting so much special attention?

There Could Be Aliens!

Okay, yes. There could be aliens.

But I doubt it. TRAPPIST-1 is a flare star. We’ve met flare stars before. You don’t want to live near one.

Also, these planets are so close to their parent star that they are almost certainly tidally locked, with one side perpetually facing the sun and the other side perpetually turned away from it. Katy Perry could write a song about how hot and cold these planets must get.

Still, it’s not impossible for life to evolve under these conditions. Just don’t get your hopes up.

An Astrophysicist’s Dream Come True

There’s still a lot I haven’t read yet about TRAPPIST-1, and no doubt there’s even more information still to come. But at this point, I’m getting the impression that this miniaturized solar system is like an astrophysicist’s dream come true. Here’s why I think that:

  • From our vantage point here on Earth, these planets pass directly in front of their star (i.e.: they “transit” their sun). This is convenient for us. It’s a lot easier to collect data about transiting planets than non-transiting ones.
  • These planets are all very close to their parent star, and therefore they all have relatively short orbital periods. That means more transits and more opportunities to collect data.
  • There are so many planets so tightly packed together that it’s easy for us to study the gravitational interactions between them.
  • And again, because these planets have short orbital periods, these gravitational interactions are sort of accelerated compared to similar interactions in our own Solar System or in other star systems we’re currently observing. I imagine these interactions are also much stronger, since the planets are so much closer together.

TRAPPIST-1 is basically a mini-solar system running on fast-forward. We can collect loads of data about it in a matter of days or weeks, rather than years or decades, and use that data to refine our current theories about solar system dynamics.

That, I think, is the real reason TRAPPIST-1 and its seven planets are such a big deal. At least that’s what’s got me the most excited about them, and why I think we’ll be hearing a lot about the TRAPPIST-1 system for many years to come.

If we happen to discover alien life there as well, that’ll just be an added bonus.

Sciency Words: Earth Similarity Index

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Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


As I’m sure you’ve heard by now, astronomers have discovered seven planets orbiting a nearby star called TRAPPIST-1. Even more exciting, most or all of these new worlds are being described as Earth-like planets. But what does that actually mean?

You’d be surprised by how many “Earth-like” planets/moons we have right here in our own Solar System.

Ag16 Earth-like Worlds
From left to right: Venus, Earth, Mars, and Titan (Saturn’s largest moon).

Earth-like is a rather vaguely defined term. So in 2011, a paper published in the journal Astrobiology attempted to establish an official mathematical system for calculating just how Earth-like an exoplanet is. It’s called the Earth Similarity Index or E.S.I.

Basically, the E.S.I. takes certain characteristics of a planet that can be quantified—such as a planet’s mass, radius, temperature, etc—and compares them to Earth’s. An E.S.I. score of zero indicates a planet that has absolutely nothing in common with Earth, while an E.S.I. of one means the planet is an exact match for Earth… at least with regard to the characteristics being measured and included in our calculations.

Of course even a planet with an E.S.I. of one is not necessarily habitable, so the same Astrobiology paper also proposes a Potential Habitability Index or P.H.I. But that, I think, is a Sciency Word for another day.

P.S.: If you want to dive into the math behind the E.S.I., click here.