Monday, March 11, 2013

Our Newest Neighbors (astronomically speaking)

Big news out of Penn State Astronomy. Dr. Kevin Luhman just published a paper in which he identified the third-closest set of objects to our solar system in the galaxy. I call this a "set of objects" rather than a "star system" because technically, it isn't a star system at all, but I'll come to that later.

Dr. Luhman specializes in studying brown dwarfs, objects that come up just short of being full-fledged stars. Brown dwarfs are not massive enough to sustain nuclear fusion reactions in the same way that stars do. While more massive brown dwarfs can fuse deuterium (a hydrogen nucleus with a neutron attached to it) and lithium early in their lives, these stages don't last for more than a billion years or so. Beyond that, brown dwarfs are basically just gravitationally bound balls of fairly cool gas ("cool" meaning between roughly 3000K and 300K, so "cool" in astronomical terms) that emit radiation because they are hot and opaque (see: Kirchoff's Laws of Spectroscopy).

Dr. Luhman works primarily on studying and cataloging brown dwarfs with data from the Wide-field Infrared Survey Explorer (WISE), a NASA satellite active between 2009 and 2011, that conducted an all-sky survey in four wavelength bands: 3.4, 4.6, 12, and 22 microns (micrometers). The reason you need infrared images for this kind of work is because brown dwarfs, as previously stated, are fairly cool (by astronomy standards), and they don't emit well in the visible part of the spectrum. However, from Wein's Law, we know that the thermal emission from a brown dwarf will peak in the infrared. So, simply put, that's just where they're best seen.

As part of its mission, WISE imaged the entire sky twice, which allows astronomers to observe regions of the sky at two different points in time. What Dr. Luhman initially saw was that, within a particular field of view, a certain object appeared to move in space over time. When he tracked down previous images of this region of the sky through the Two-Micron All-Sky Survey (2MASS) and the Digitized Sky Survey (DSS), he saw more of the same. The object was moving very quickly across the field of view, or (in astro-jargon) it had a high proper motion. The image below shows exactly what Dr. Luhman saw.
WISE J104915.57-531906 was discovered through its rapid motion across the sky, which is shown in these images taken between 1978 and 2010 by the DSS, 2MASS, and the WISE satellite. Credit: NASA/STScI/JPL/IPAC/University of Massachusetts.
Pretty cool, huh?

Well, it gets better. Based on the motion of the object, that we see here, Dr. Luhman also determined the object's distance from the solar system through astrometry, finding that it was 2.0±.15 parsecs (6.5±.49 lightyears) away. This makes WISE J104915.57-531906 (don't worry about the name too much, it's just based on the object's coordinates) the third-closest object to Earth behind the Alpha Centauri system and Barnard's Star, shown schematically in the image below.
This diagram shows the locations of the three closest star systems to the Sun with the year of discovery indicated along with the name of the system. WISE J104915.57-531906 is the third-nearest system to the Sun, and the closest system discovered in nearly a century. Credit: Janella Williams, Penn State University.

But wait, there's more! When doing follow-up observations of the object with the Gemini Multi-Object Spectrometer (GMOS) on the 8.4 meter Gemini South telescope, Dr. Luhman saw that the "object" was actually a binary system consisting of two brown dwarfs. In a binary system, the two objects are gravitationally bound to one another and orbit a common center of mass (like Earth and its Moon, but bigger in this case). These two brown dwarfs happen to be orbiting at a distance of roughly 3 astronomical units (AU, the mean distance between Earth and the Sun) from one another. The high-resolution image recorded by GMOS compared to that from WISE is also shown below.

WISE J104915.57-531906 is at the center of the larger image, which was taken by WISE. Inset is the image from Gemini Observatory, revealing the object to be a binary system. Credit NASA/JPL/Gemini Observatory/AURA/NSF

I may not be a brown dwarf aficionado, but I gotta admit that this is pretty cool. I mean, astronomically speaking, this system is right in our own backyard, and it took us this long to properly identify it. We have every reason to believe that many more systems like this could be lurking out there just waiting for us to stumble across them.

Keep it up, Kevin!

For anyone interested, a copy of Dr. Luhman's publication is available here.


  1. Hello! Looking forward to seeing what you do with this space -- I got here via Steinn's Dynamics of Cats blog.

    Does the non-announcement (so far) of a detection of a brown dwarf closer to us than Alpha Centauri allow us to infer that such a detection isn't likely to be forthcoming anytime soon? I've read elsewhere on the web that it looks like WISE didn't find any such thing, but Kevin Luhman's paper leads me think that WISE could very well have detected such an object but it will go unnoticed because there is too much data to sort through.

    Also, if a close BD exists but it doesn't happen to have much proper motion (ie it is heading toward us, away from us, or has matched course with us), it is even less likely to be detected -- is that correct?

    On Steinn's blog, there is talk about the likelihood of finding a brown dwarf in any given volumn of space, but I'm not asking about that -- I'm asking about the mechanics of using the data from WISE. Since a closer BD hasn't been found already, does that suggest that one isn't likely to be found, or is that WISE dataset just too vast to suggest any such thing.

    Thanks for any guesses on the prospects for closer objects being found via the WISE dataset.

    1. Welcome! I will preface my response by saying right off that I am certainly not an expert in this area whatsoever. I have also not worked with WISE data, so anything I say about that is speculative

      1) I'd say that seems a reasonable conclusion, but you can never really count anything out it seems. Do I personally think that there could be much that is closer to us than the Alpha Centauri system? Not really. But yes, there is a ton of WISE data to sort through, and only by noticing weird things like Kevin did with this brown dwarf system over time will we confidently be able to say that there is or isn't anything else in the immediate neighborhood of the solar system closer than Alpha Cen. One of the main problems with survey-like experiments is the sheer volume of data you get from them. And there are only so many of us grad students to go around!

      2) That sounds somewhat reasonable. If you don't see high proper motions, then it makes sense that a brown dwarf wouldn't stand out as easily in observational data. Further, WISE has a resolution of around 6 arcseconds. So it would be unlikely to see any parallax from such a source, unless it is within about a lightyear of us.

      Trust Steinn to do a back of the envelope likelihood calculation...

      Thanks for sticking your virtual head in!