Sunday, June 26, 2016

An Exceptionally Low-Mass “Brown Dwarf”

Free-floating, planetary-mass objects are predicted to be quite common in the galaxy. These objects hide in the darkness between stars and may even outnumber stars themselves. Using data from NASA’s Wide-field Infrared Survey Explorer (WISE) and the Two Micron All Sky Survey (2MASS), Schneider et al. (2016) present the discovery of a free-floating, planetary-mass object identified as WISEA 1147-2040. This object is located ~100 light years away and it is part of a group of young stars known as the TW Hydrae association. Its membership in the TW Hydrae association means that it is a young object, only ~10 million years old.

Observations of WISEA 1147-2040 also provided additionally evidence consistent with its youth. For example, WISEA 1147-2040 is observed to have an unusually dusty atmosphere, and this indicates it has a relatively low surface gravity. A young brown dwarf like WISEA 1147-2040 is still in the process of cooling and contracting to its final radius. As a consequence, it is still somewhat “inflated”, giving rise to its relatively low surface gravity. WISEA 1147-2040 is estimated to be between 5 to 13 times the mass of Jupiter. Furthermore, its effective temperature is predicted to be ~1100 to 1200 K.

WISEA 1147-2040 most likely formed as a brown dwarf rather than a planet that got ejected from its natal planetary system. This is because WISEA 1147-2040 is only ~10 million years old, and such a span of time is probably too short for a planet to form and subsequently get ejected. Brown dwarfs are objects that formed in the same way as stars do, but they lack the mass to fuse hydrogen in their cores. Observing these isolated, planetary-mass objects can provide greater insights to the characteristics of gas giant planet around other stars. This is because, unlike planets, these isolated objects are much easier to study as they are not lost in the glare of a host star.

Schneider et al. (2016), “WISEA J114724.10-204021.3: A Free-Floating Planetary Mass Member of the TW Hya Association”, arXiv:1603.07985 [astro-ph.SR]