Knowing the position of a firefly within one inch from a distance of 200 miles would not be easy, but it’s the kind of precision astronomers Adam Kraus and Trent Dupuy needed when trying to establish the distance of nearby brown dwarfs. The firefly simile belongs to Kraus (University of Texas at Austin), who with Dupuy (Harvard-Smithsonian Center for Astrophysics) embarked on a study of the initial sample of the coldest brown dwarfs discovered by the Wide-Field Infrared Survey Explorer satellite (WISE). Their paper appears today is Science Express online.


Image: Brown dwarfs in relation to more familiar celestial objects. Credit: Gemini Observatory/Jon Lomberg.

Just how cool can brown dwarfs get? When we’re focusing on small dwarfs somewhere between 5 and 20 times the mass of Jupiter that have been cooling for billions of years, we’re talking about objects whose only source of energy is gravitational contraction, and as Dupuy notes, the fine-grained distinctions between star and planet begin to get blurred here: “If one of these objects was found orbiting a star, there is a good chance that it would be called a planet.”

So what does make the difference? The key determinant is that brown dwarfs formed on their own rather than in a proto-planetary disk. Moreover, they’re exceedingly hard to characterize because most of their light is emitted at infrared wavelengths and their small size and low temperature make finding them tricky. The astronomers, in Dupuy’s words, “wanted to find out if they were colder, fainter and nearby or if they were warmer, brighter and more distant.”

To answer these questions involved measuring their distance accurately. For that, the duo turned to the Spitzer Space Telescope and put parallax methods to work on the nearby brown dwarfs previously identified by WISE. We’ve often discussed parallax in these pages as the method used to make measurements of stellar distance, a feat first accomplished by the German astronomer Friedrich Wilhelm Bessel in 1842 with his work on 61 Cygni — Bessel’s reading of 10.3 light years wasn’t all that far off the now accepted value of 11.4 light years.

The early days of parallax in astronomy (and there were 60 stellar parallaxes in the literature by 1900) involved observing a star from one side of the Sun’s orbit and then, half a year later, the other, looking for the slight changes that would make the calculation possible. In the case of Kraus and Dupuy’s work with the Spitzer instrument, the needed precision was mind-boggling but measurable, enough to determine that the objects in question range between 20 and 50 light years away.

The upshot: Brown dwarf temperatures in the range of 395 to 450 K (250 to 350 degrees Fahrenheit), allowing them to retain their status as the coldest known free-floating celestial bodies but making them a bit warmer than some earlier studies have suggested. These objects cool slowly over time, their heat produced by contraction rather than fusion. Recent work has suggested that brown dwarfs could sustain habitable conditions for tightly orbiting planets for several billion years. I mention this because yesterday we saw in the work of Mukremin Kilic that planets in certain configurations around white dwarfs could have eight billion years of habitability.

What lies ahead is the great hunt to discover whether worlds like these actually exist. What a time to enter exoplanet studies — a theme we keep encountering is that habitable conditions may exist around stars far different from our Sun. M-dwarfs first opened our eyes to this but brown and white dwarfs push us into whole new realms of investigation as we try to learn how frequently planets can form around them and whether any might have astrobiological possibilities. Not so long ago we assumed that other solar systems would look more or less like ours. Now we look into a cosmos filled with exotica and ask whether we’re not the outliers.

For more on brown dwarf planets and habitability, see Andreeshchev and Scalo, “Habitability of Brown Dwarf Planets,” Bioastronomy 2002: Life Among the Stars. IAU Symposium, Vol. 213, 2004 (abstract), as discussed in Brown Dwarf Planets and Habitability. See also this Astrobiology Magazine feature on why white and brown dwarf planets may not be capable of sustaining life (thanks to David Cummings for the reference to this one).