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SYFY WIRE Bad Astronomy

Three Earth-Size Planets Found Orbiting a Nearby Ultracool Star

By Phil Plait

The search for Earth-like planets continues, as astronomers scour the sky examining stars for telltale clues of orbiting worlds. Most of the exoplanets found are big, like Jupiter, because they’re the easiest to detect. But our technology has become better and more clever over the years, and smaller planets have been found, including many roughly the size of Earth.

That search took a cool turn this week … literally. A team of astronomers announced they have found not one but three Earth-size planets orbiting a red dwarf, a tiny and cool star just 40 light-years away!

This is very interesting for many reasons: This is the lowest mass full-fledged star ever seen to have planets, it’s relatively close by, and all three planets are (more or less) in the star’s “habitable zone,” where temperatures might—might—support the existence of liquid water on the planets’ surfaces.

The European Southern Observatory put together a nice video explaining this, so give it a view:

The planets were discovered using TRAPPIST (short for Transiting Planets and Planetesimals Small Telescope). This is a 60 cm (24”) telescope that takes images of a select group of 60 nearby red dwarf stars visible from the Southern Hemisphere. The team looks for dips in the stars’ light that are caused by any planets orbiting those stars periodically blocking their host star’s light; this is called the transit method, and most exoplanets have been discovered this way.

TRAPPIST found evidence of planets orbiting a star, called TRAPPIST-1, and follow-up observations were made with much larger telescopes. Three planets were found in total, which is remarkable all by itself. But it gets better.

TRAPPIST-1 is an M8 dwarf, only 0.08 times the mass of the Sun; just barely massive enough to fuse hydrogen into helium in its core. If it were much lower mass we wouldn’t call it a star at all (we’d say it’s a brown dwarf). Its surface temperature is only about 2,550 K—the Sun is literally more than twice as hot—so it's informally called an "ultracool" star. And it’s tiny, only about 0.11 times the diameter of the Sun. That’s roughly the same size as Jupiter!

Yet this teeny star sports at least three planets. Called TRAPPIST-1b, c, and d, the exoplanets were detected as they blocked a small fraction of the star’s light. The sizes of the planets were found by seeing just how much of the starlight they blocked. The best measurements indicate they are 1.1, 1.05, and 1.2 times the size of Earth. We don’t know their masses, but if they have the same composition as our home world—rock and metal—then their surface gravities wouldn’t be all that different than ours.

So they’re Earth-size. But are they Earth-like? That is, nearly the same temperature and composition as Earth?

We have no idea what these planets are made of. They could be rock, metal, watery, airless … with our current technology we don’t know how to determine that. Finding the masses of these planets would be extremely difficult, so we’re out of luck there.

But we can estimate their temperatures. The temperature of a planet depends on its distance from the star and that star’s temperature, of course, but also on how reflective the planet is; a more reflective planet will be cooler than a dark, absorptive one.

Each of the planets orbits ridiculously close to the star compared with the planets in our solar system. In order, they’re 1.7 million, 2.3 million, and 3.3 million–22 million kilometers from the star (the observations of the third planet, d, don’t constrain its distance very well, so there’s a range of possible distances). Mind you, Mercury is 58 million kilometers from the Sun, so all three of these planets would easily fit inside Mercury’s orbit, with a tens of millions of kilometers to spare.

But remember, the star is very cool, so even at those distance the planets aren’t as hot as you might think.

Assuming very dark planets, the inner two would be about 125° and 70° C, far too hot for life as we know it. The outer planet’s distance from the star wasn’t as well determined, but it would likely have a temperature somewhere from -160° - +10° C depending on its distance. The warmer end of that range is close to Earth’s average temperature!

Remember, that’s assuming dark planets. If they’re more like Earth (which reflects about 40 percent of the light that hits it), they’ll be cooler. If they’re reflective enough, the inner two planets might be more like Earth, too (but the outer planet would be a frozen ball of ice).

That part is more speculative; we have no idea how reflective they are. It’s possible, though I’d think unlikely, that all three planets are somewhat clement.

Again, we don’t know much about them; they might be airless, or have thick atmospheres of carbon dioxide, or some other noxious combination, so don’t start looking into real estate on them just yet. And even if they are Earth-like, 40 light-years is 400 trillion kilometers. That’s a fairly long road trip. It would take 450 million years to drive there at highway speeds. Better pack a lunch.

But don’t be disappointed. The amazing thing to remember here is that these planets exist at all. Even dinky red dwarf stars manage to make planets, including ones the same size as ours. That’s incredibly exciting.

Another reason this is so exciting is because of the host star. Cool red dwarfs are faint and hard to detect, making these observations somewhat difficult, but they also make up the most populous class of star in the galaxy. If they have planets in the same proportion as more massive, hotter stars do, then planets orbiting red dwarfs will outnumber planets orbiting all other types of stars combined. And here we found three Earth-size planets orbiting one nearby.

It’s impossible not to ask, how many planets like Earth exist in the galaxy? We’re not sure, but various methods have been used to estimate that number, and even conservatively their numbers must be in the billions. Billions. In our galaxy alone.

And our tech is getting better. In the coming years we’ll have telescopes able to dissect the light from such planets, looking for the Earth-like conditions: oxygen in the atmosphere, say, and a temperature more like ours. We’ve found a few candidates for Earth-like exoplanets, but nothing yet that we can point to and confidently say, “Earth 2.”

Nothing yet.

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