News of the remarkable density of planets, including potentially Earth-like planets, in the system of nearby ultra-cool dwarf TRAPPIST-1 spread across the blogosphere. This NASA JPL illustration comparing the TRAPPIST-1 worlds with the four rocky worlds of our own solar system, underlining the potential similarity of some worlds to the worlds we know like Venus and Mars and even Earth, went viral.
One interesting thing is that TRAPPIST-1 is tiny. Really tiny! It’s a class M8V ultracool red dwarf, which really is about as small as a star can get while still being a star. Much smaller and it wouldn’t be able to even fuse hydrogen. I’ve put it side by side with a few other familiar celestial objects in this image. As you can see, it’s a little bigger than Jupiter. It’s actually roughly the same size as HD189733b, a much studied hot jupiter, and noticeably smaller than Proxima, our friendly neighbourhood red dwarf. Lalande 21185 is on the larger end of the scale of red dwarfs, and is also one of the few you can actually see in the night sky (though you’ll need a dark sky to find it).
Ultracool red dwarfs really are tiny, but they’re also extremely long lived. Quietly burning stellar embers which exemplify the old saying that slow and steady wins the race. Because these little stars don’t burn their fuel too quickly, and because they’re low enough in mass to be fully convective, they can burn for trillions of years. Long after the Sun exhausts the fuel in its core, flares into a red giant and then cools silently in the darkness, TRAPPIST-1 will still be burning, providing warmth for it’s little planetary entourage.
Not much warmth, mind you. TRAPPIST-1’s handful of planets are huddling around their parent star as if it were campfire on a cold night. The entire star system would fit inside Mercury’s orbit and still have cavernous amounts of room to spare. So close are those planets, that they have years which pass by in mere Earth days. The shortest has a year which is just 1.5 Earth days long. The longest year length in the system is still less than a month.
aureliaOf course, I say Earth days, because these planets don’t have days as such. They’re so close to their parent star that they’re certain to be tidally locked. The gravitational forces are sufficiently different that they cannot rotate at all. One side constantly faces the tiny red sun in the sky, and the other side constantly faces outwards towards the cold night. It’s quite likely that the night sides of these planets may be frozen in a permanent winter night, never gaining enough warmth to thaw. Half a planet of permanent Antarctica.
Supernova Condensate was kind enough to produce an illuminating graphic, hosted at “Model Planets”, comparing the TRAPPIST-1 system to (among others) the Earth-Moon system and to Jupiter and its moons. The TRAPPIST-1 system is tiny.
The Planetary Society Blog’s Franck Marchis wrote a nice essay outlining what is and is not known, perhaps most importantly pointing out that while several of the TRAPPIST-1 worlds are in roughly the right position in their solar system to support life, we do not actually know if they do support life. Further research is called for, clearly.
Centauri Dreams’ “Seven Planets Around TRAPPIST-1” has great discussion in the comments, concentrating on the potential for life on these worlds and on the possibility of actually travelling to the TRAPPIST-1 solar system. The later post “Further Thoughts on TRAPPIST-1” notes that these worlds, which presumably migrated inwards from the outer fringes of their solar system, might well have arrived with substantial stocks of volatiles like water. If this survived the radiation of their young and active sun, they could be watery worlds.
The cultural implication of these discoveries, meanwhile, has also come up. Jonathan Edelstein has written in “We Just Got Our ’30s Sci-Fi Plots Back” about how TRAPPIST-1, by providing so many potentially habitable planets so close to each other, would be an ideal setting for an early spacefaring civilization, and for imaginings of said. If a sister world is scarcely further than the moon, why not head there? Savage Minds, meanwhile, in “The Resonance of Earth, Other Worlds, and Exoplanets”, hosts a discussion between Michael P. Oman-Reagan and Lisa Messeri talking about the cultural significance of these and other discoveries, particularly exploring how they create points of perceived similarity used as markers of cultural import.
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