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SYFY WIRE Bad Astronomy

Astronomer Finds a Brown Dwarf Literally as Cold as Ice … and It’s Right
Next Door

By Phil Plait

Stars are hot, right? You might think that’s one of their defining characteristics.

But that’s not entirely true. Brown dwarfs are star-like objects that are more massive than planets, but not quite massive enough to ignite sustained fusion in their cores. Hydrogen fusion is what powers the Sun, and makes it hot; it’s the mighty pressure of the Sun’s core that makes that happen. Brown dwarfs don’t have the oomph needed to keep that going.*

Brown dwarfs are born hot, then cool over time. And now one has been found that is literally as cold as ice. Not only that, but it’s very close to our solar system: Just 7.2 light years away! That makes it the seventh closest known star to the Sun.

The object is called WISE J085510.83-071442.5. I know, I know, but it’s given that designation based on the fact that it was discovered in data taken by the Wide-field Infrared Survey Explorer, with the rest of the number salad based on its coordinates on the sky, so let’s call it J0855 for short. WISE observed the entire sky several times over its short 13-month lifetime, looking at cooler and downright cold objects in the Universe. It saw stars, dust clouds, galaxies … and brown dwarfs. Hundreds of brown dwarfs, in fact.

One of them caught the eye of Kevin Luhman, an astronomer who specializes in them. This one was very red and very faint— as you’d expect for a very cold object—but it was also moving. Fast. Stars aren’t stationary; they actually move around the sky as they each follow their orbits around the center of the Milky Way galaxy. That motion in space is usually a few hundred kilometers per second, but stars are so far away their motion across the sky is pretty slow. It could take centuries to see some of them move noticeably. But if a star is close, very close, to the Sun, that motion can be detected in just a few years, or even months.

[Actual observations of J0855 by WISE and Spitzer show it moving over time (Spitzer has sharper resolution and better vision, so more stars are seen and appear smaller than in the WISE images). Photo by NASA/JPL-Caltech/Penn State]

J0855 moved quite a bit between May and November 2010 when WISE saw it, and Luhman figured that meant it was close. It also showed significant parallax, a change in position that reflects the Earth’s orbit around the Sun. By measuring these two motions, Luhman was able to determine the brown dwarf was a mere 7.2 light years away. There are only six stars closer than that to the Sun that we know of, so this object really is nearby as these things go.

He was able to determine its temperature by measuring how much light it gave off in different colors; hotter objects are bluer, and colder ones red. This object is so cold it’s incredibly faint even in the near infrared; he had to look at even longer wavelengths to get a temperature. It was even invisible to the massive Gemini telescope in Hawaii! He was able to spot it using the Spitzer Space Telescope, though, and was able to nail down its temperature. And this is the part that kills me: the best fit temperature he found was around 225–260 Kelvins. Even at the high end, that’s -13° C (9° F). That’s literally colder than ice (or at least the freezing point of water). That’s barely warmer than the temperature of the freezer in my kitchen.

That’s incredible. It implies this object is very old, too, because it would’ve been a few thousands degrees when it formed, and would take at least a billion years to cool down to its current chilly temperature. It’s hard to determine how old it actually is, but it’s most likely 1-10 billion years old.

It has a very low mass, too, probably between 3 and 10 times the mass of Jupiter. That’s pretty lightweight even for a brown dwarf. And here’s another amazing thing about it: It might be a planet. What I mean is, it may have formed around a star like a planet does, then got ejected by gravitational interactions with other planets. If so, it was kicked out of its solar system, doomed to wander the galaxy on its own as a rogue planet. We know such objects exist, and there must be many billions of them in deep, cold space.

However, there’s no way for us to really know, at least not until we have the Enterprise to take us there and get a close look (72 trillion kilometers is still a dang long walk).

Ever since the first brown dwarf was discovered in 1995, I’ve wondered if there might actually be one closer to us than even Proxima Centauri, the closest known star. It would have to be very cold indeed to have escaped our notice, but the existence of J0855 makes me wonder. It’s the seventh closest star and it was only just discovered! Could there be fainter objects—maybe one of those rogue planets—even closer?

Maybe. We’ll know better as we build bigger and more sensitive infrared observatories in space, and let them scan the skies. We may yet be in for a pretty big—and literally cool—surprise.

*Some people might argue that because brown dwarfs can’t sustain fusion in their stars they aren’t really stars. As usual, when you get near the borders of definitions things get fuzzy, and definitions become less than useful. Some people think brown dwarfs are more like planets, and other think they’re more like stars. I think it’s best not to let ourselves get boxed in with arbitrary definitions, and to just let brown dwarfs be brown dwarfs. I generally call them “objects” when I'm trying to be generic, but it's not awful or terribly incorrect to call them stars as long as you keep that in mind.

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