Brown dwarfs

Brown dwarfs and exoplanets

Brown dwarfs are a class of objects halfway between giant planets, like Jupiter, and stars. All objects with a mass greater than about 75 times that of Jupiter (7% of the Sun’s mass) are stars, and mainly burn hydrogen for their entire lifetimes. During this phase of peaceful nuclear combustion, their luminosity remains quite stable. Low-mass stars (less than 50% of the Sun’s mass) conserve the same flux for tens of billions of years – much more than the current age of the universe.

Objects with a mass below this threshold (75 times Jupiter’s mass) thus inexorably cool, starting from a temperature of about 3000 °C, to reach ambient room temperature in a few billion years. Lighter objects (less than 13 times Jupiter’s mass) are called (giant) planets, while more massive ones (13-75 times Jupiter’s mass) belong to the brown dwarf class.

Aside from this apparently arbitrary subdivision, brown dwarfs can be distinguished from giant planets in some other ways. For example, there are very few brown dwarfs in close orbit around stars, while this is very common for planets. In addition, there are many brown dwarfs in the solar neighbourhood (1 star out of 6 is a brown dwarf), while “orphan” (starless) planets appear to be much rarer. CFBDSIR2149, a planet with no parent star, recently discovered by researchers at the Institute in collaboration with French researchers, has blurred the line between planets and brown dwarfs, however.

Generally speaking, brown dwarfs are isolated stars, and it is relatively easy to obtain a detailed spectrum for them. This is not the case for exoplanets located very (too) near their parent stars. Since brown dwarfs cool in the same way as giant planets and many physical phenomena governing their atmospheres are common to both, studying brown dwarfs is a way of constructing and testing atmospheric models that can also be used to study exoplanets. The Institute team has conducted and taken part in various studies of brown dwarfs, in particular efforts to detect brown dwarfs in the solar neighbourhood (e.g. 123456), study their variability (e.g. 1234), discover brown dwarfs in binary systems, both star/brown dwarf and brown dwarf/brown dwarf (e.g. 1234), and discover young brown dwarfs (e.g. 123).

Cloudy with occasional breaks

Work at the Observatoire du Mont-Mégantic made it possible to detect the signature of clouds on the surface of a cold brown dwarf for the first time. Brown dwarfs have relatively short rotation periods, on the order of several hours. If clouds are distributed irregularly on the surface of a brown dwarf, it is possible to observe periodic variations in brightness as it rotates.

Figure 1. SIMP0136+0933 flux during 4 nights over 1 week. Variability is clearly apparent, a sign of the presence of clouds on the surface of this 900 °C brown dwarf.

By observing the brown dwarf SIMP0136+0933 for several nights, an object with a temperature of 900 °C, we managed to detect a variation in its flux every 2.4 hours, indicating the presence of clouds. In addition, the form of the light curve evolved from one night to another (see Figure 1), suggesting that the cloud models at the surface of SIMP0136+0933 change shape after several days. This discovery, published in 2009, led researchers to look for similar variability on other objects, in collaboration with researchers from Toronto and elsewhere (e.g. 12). These results allowed us to determine that the variability of cold brown dwarfs comes from breaks in the dust clouds (titanium oxide, iron hydride), revealing occasional much warmer and dust-free regions beneath the clouds.

This type of phenomenon has not yet been observed on giant planets, but it is probable that instruments on board the JWST telescope will make it possible to detect the signature of cloud patterns on the surface of giant planets like those of HR8799.