You can't take the sky from me: Firefly vs. TRAPPIST-1

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Mar 20, 2017

About 40 light years from Earth, there is a solar system with dozens of Earths and hundreds of moons.

Well, at least there is in the Firefly Universe.

Strangely enough, here in our own universe, we recently discovered a solar system about 40 light years from Earth with over half a dozen Earths. So while we might not be anywhere close to living in the Firefly Universe (despite my continued pleas to the Joss Whedon gods), seven Earth-sized planets in one system is a pretty good start.

Last month, an international team of astronomers announced they had identified four new Earth-sized planets (in addition to three already known) orbiting a red dwarf star in the direction of the constellation Aquarius. Not only that, but three of the seven planets in this system are within the star's habitable zone. If there were ever a Goldilocks lottery for planets, all three of these would have better odds than most.

This exciting new planetary system is called TRAPPIST-1 after the telescope that detected the first three planets (TRAPPIST-1b, TRAPPIST-1c, and TRAPPIST-1d) around the star in 2015: the Transiting Planets and Planetesimals Small Telescope at the La Silla Observatory in Chile. Exoplanets are assigned letters of the alphabet based on the order of their discovery (starting with "b" because the host star is technically "a"), so the four new planets are TRAPPIST- 1e, TRAPPIST-1f, TRAPPIST-1g, and TRAPPIST-1h, making this the fourth 7-planet system we've found to date.

firefly so fancy

As soon as I heard about TRAPPIST-1, my nerdy brain went straight to Firefly since I'm obsessed with the idea of a star and/or planetary system with multiple, nay many, habitable worlds. So I decided to look into a bit more, because science (and shiny!).

Okay, technically the Firefly system is actually a five star cluster, but I wanted to see if any of them could come close to being TRAPPIST-1-esque. After all, one of them is actually called the "Red Sun" and the star these new exoplanets orbit is classified as an M8 red dwarf. On the scale of stars, that means it's smaller and dimmer than our Sun -- much, much smaller actually, and much, much dimmer. It's only 10% the radius of the Sun, and 8% of its mass, meaning it's less than 1% as bright.

When you look at the numbers, though, not only is the Red Sun more massive than any red dwarf star, all five stars in the system are. The Red Sun is quite sun-like which is weird because for a star to appear red in color, it should either be a massive red giant or a diminutive red dwarf. There's also a disconnect with the also sun-like "Blue Sun" in the Firefly system. Blue stars in our universe are not only huge compared to our sun, they are almost ten times hotter. Confused astronomer is confused.

Moving on, I looked at planetary orbits next. Since all the stars in Firefly are sun-like to a degree, their habitable zones are similar to that of our solar system. Habitable zones are defined by the distance from a star where the temperature on the surface of a planet would allow for liquid water (i.e., between 0 and 100 degrees Celsius). The hotter the star, the farther out the habitable zone; the colder the star, the closer in. Given that TRAPPIST-1 is a red dwarf, it's habitable zone is uber close in.

How close? Like uncomfortably close, invading your personal space close. A year (one orbit) in the TRAPPIST-1 system ranges from 1.5 days to roughly 20 days, depending on the planet. This system has more in common with Jupiter where the Galilean Moons whip around the gas giant on similar timeframes to the TRAPPIST-1 worlds than our Sun where a year on Mercury is 88 days.

Jupiter is a planet, though, not a star. Liquid water could exist on more than one of the Galilean Moons, but it would be due to heating by tidal friction, not by radiation. However, there are objects in the universe tens of times more massive than Jupiter but not yet massive enough to be stars that can radiate like some low mass stars, enough to nurture a planetary system for hundreds of millions, if not billions, of years. We call these objects brown dwarfs there happen to be five of them in the Firefly universe.

Sadly, I was foiled yet again by the Whedonverse. In the Firefly backstory, pieced together by fans (in oh so much awesome detail), all five brown dwarfs were "helioformed" meaning they were compressed enough to ignite fusion in their cores and boom! Instant star.

Even before their transformation though, they were still several times more massive than TRAPPIST-1, which means that post-transformation, their habitable zones were almost a million times farther out than all seven of the TRAPPIST-1 planets. Since Firefly was written a decade and a half ago, I wonder if the exoplanet systems know at the time played a role in limiting the imagination of the writers, despite how far they appear to have stretched it. At the time, we were mostly finding Jupiter-sized planets super close in to their host stars, and the technology for finding Earth-sized planets was just out of reach.

But now here we are in 2017. Within two years, we have the first detection of an Earth-sized planet in the habitable zone of a sun-like star and the first detection of a seven Earth-sized planet system. Maybe truth is stranger than fiction after all.