WISE: Brown Dwarf Hunter Extraordinaire

Friday is a travel day for me, so be aware that comment moderation will be slow and sporadic. I just have time to get in word about the upcoming launch of the WISE mission, slated for December 7. NASA is planning a media briefing next Tuesday (November 17) to discuss the mission, which is designed to scan the entire sky at infrared wavelengths, spotting perhaps hundreds of thousands of asteroids and studying a wide range of stars and galaxies.

The technology is fascinating in and of itself. WISE will image the entire sky in the infrared, using detectors kept below 15 Kelvins (which is only 15 degrees C above absolute zero) by a solid hydrogen cryostat. The telescope will be oriented to look out at right angles to the Sun, always pointing away from the Earth, so that its observations sweep out a circle in the sky. After six months, the instrument will have observed the entire sky, producing nearly 1.5 million images and creating, ultimately, an atlas of the entire celestial sphere.

This is exciting stuff. For one thing, WISE should be able to measure the diameters of more than 100,000 asteroids. For another (and this may be of the most interest to Centauri Dreams readers), WISE will be able to detect stars much dimmer than the Sun. These brown dwarfs, many of which have yet to be discovered, should be readily apparent to the WISE instrument, and of course we hope for one that ranks as the closest star to the Earth. And beyond all this, WISE will be able to produce a global map of the galaxy and its associated dust.

But back to the brown dwarf issue. WISE principle investigator Ned Wright refers to the chart below. WISE is sensitive to radiation with wavelengths of five microns, useful for our purpose because from brown dwarfs down to Jupiter-class gas giants, a large fraction of the emitted radiation appears at five microns, as the figure shows:

About which Wright has this to say on a page of his Web site devoted to the brown dwarf hunt:

These low mass stars are expected to be more numerous than the more massive stars like red dwarfs, and thus there should be brown dwarf stars closer to the Solar system than Proxima Centauri [italics mine]. The green curve shows a 200 K model atmosphere calculation from Burrows et al. (1997) for an object with the radius of Jupiter at the distance of Proxima Centauri. WISE will easily be able to detect these nearby brown dwarfs.

Image: Plot showing nearby objects compared to the sensitivity of WISE. A free floating Jupiter at 1 light year (FFP), and a 200 K brown dwarf at the distance of Proxima Centauri (BD). Credit: Edward Wright/UCLA.

So if we are dealing with a brown dwarf closer than any other star, WISE ought to be the mission to find it. Surveys like the Two-Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) have discovered numerous brown dwarfs, but have been unable to locate any cooler than 750 K. We can expect WISE to see 450-K brown dwarfs out to a distance of 75 light years, and brown dwarfs as cool as 150-K out as far as ten light years. All eyes may be on Kepler and CoRoT for terrestrial exoplanets, but a nearby brown dwarf would be huge, putting WISE on the front pages.