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

September 12, 2017

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

March 13, 2017

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

March 10, 2017

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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

March 8, 2017

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

March 3, 2017

Sciency Words BIO copy

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.