A record-breaking brown dwarf: as cold as ice and just 6 lightyears away

About a year ago I wrote about how Kevin Luhman at Penn State had discovered a pair of brown dwarfs that were the just 2 parsecs (about 6 lightyears) from the Sun. Well he’s gone and done it again, discovering another brown dwarf at about 2pc, only this time it’s colder, much, much colder.

A decade and half ago, we astronomers (being rather odd) were getting very excited about some new odd objects we were finding by looking at the sky in infrared light. These brown dwarfs bridged the gap between very low mass stars (which can go down to about 8% of the mass of the Sun) and giant planets like Jupiter (with a mass of about 0.1% of the mass of the Sun). Brown dwarfs can’t fuse hydrogen in their cores so they don’t have a stable brightness like stars do and hence cool with time. This means that a very cold brown dwarf could be very low mass or just very old. Anyway, the things we were getting excited about 15 years ago had temperatures of about 1100C. At this point the cloud physics of these objects change dramatically, their upper atmospheres clear and their colours in near-infrared light changes significantly.

A decade or so went along and we started to get more excited as we crept to lower and lower temperatures, getting down to about 400-500C. Then we got a stroke of luck, Kevin Luhman (yup, same bloke) published the discovery of a really cold brown dwarf around a dead star called a white dwarf. This has a temperature of 25-80C, so between a pleasant summers day and a hot cup of tea. This object was joined by a few other slightly hotter objects which formed a newly defined class of cold brown dwarfs, the Y dwarfs. These big balls of gas about the size of Jupiter could have water clouds in their atmospheres.

So now we have a new coldest brown dwarf. It was found by looking at images from the WISE satellite which studies the universe in mid-infrared radiation. Nearby stars and brown dwarfs move slowly across the sky compared to background stars due to proper motion. This can be pretty slow, a very nearby star might move at one arcsecond per year, about the apparent angular speed of a tortoise walking at the distance of the Sun. So Luhman looked for objects that had moved a lot between different WISE images and found one which he published last year. This was a pair of cool brown dwarfs with temperatures of about 1100C. Now he’s published another that is moving even faster, about 8 arcseconds per year. Despite this, it is about the same distance as the previously published one, 2.2pc (a bit more than 6 lightyears). This distance was determined by follow-up Spitzer Space Telescope observations using a trick called trigonometric parallax.

So what is this thing? Well, we know it is bright in the mid-infrared, light which it is difficult to observe from Earth and which is way beyond what the human eye can see. And that’s where the observations of it stop, well not really, we can tell a bit about this object from what we don’t see, near-infrared light. Luhman’s new object was observed by the VISTA telescope in Chile a few years back. Well I say observed, it didn’t see it, neither did Luhman’s follow-up observations with Gemini. But from those observations one can set a limit of how bright this object is in the near-infrared and hence constrain its properties. Luhman used these along with his measurement of how bright the object was in the mid-infrared to find that the temperature was -48 to -13C, colder than ice on Earth, you’d even struggle to play at Lambeau Field in those temperatures. Not that this is a solid, icy planet, it’s about 3 to 10 times the mass of Jupiter and about the same size. It’s also a bit warmer than Jupiter which has an effective temperature at the top of its clouds of about -160C.

What more will we find out about it? Who knows. Last year’s spectacular Luhman discovery sent astronomers into a frenzy, studying the weather on the objects, even mapping its clouds. This one will be harder as the object is so cold and faint, but I’m sure observers will be furiously writing proposals to observe this immediately. Wait, why am I blogging? I should be proposal writing. And I’m sure this object will be one of the first things the mid-infrared JWST will look at when it launches.