A team made up of members of the general public and of researchers in astrophysics has just discovered two brown dwarfs with very strange properties. Brown dwarfs are celestial bodies that closely resemble giant gaseous planets like Jupiter in our solar system. However, they’re heavier than Jupiter, with masses up to 75 times greater, and they’re not necessarily in orbit around a star. They’re sometimes called super-Jupiters for that reason. Brown dwarfs are a lot colder than stars, and thus have to be studied with infrared cameras.
Brown dwarfs are in fact halfway between planets and stars: they form in a similar way, namely from an interstellar cloud of gas that undergoes gravitational collapse. When a gas fragment accumulates over 75 times the mass of Jupiter, nuclear reactions begin at its core, enabling it to become a hot, bright star. Otherwise, what we’ll be dealing with is a brown dwarf. Brown dwarfs start their lives with a temperature below 2500°C and get colder as they age. These are very cold temperatures compared with those of stars, which can reach 45,000°C! We even know about a brown dwarf, WISE 0855-0714, so light and old that its temperature is down around 0°C. We think that precipitations in the form of snow exist in its upper atmosphere.
A physical simulation of a star-formation event. We see an interstellar cloud of gas collapsing onto itself along filaments to form hundreds of stars. The colder regions of gas are shown in blue, and the warmer regions in red. Credit: Matthew Bate, University of Exeter.
It was the project Backyard Worlds: Planet 9 that led to the discovery of the two rather strange brown dwarfs, which are named WISEA J041451.67−585456.7 and WISEA J181006.18−101000.5. This project brings together images from NASA’s WISE space telescope. That observatory took pictures of the whole sky with its infrared “eyes” many times between the years 2010 and 2019. The Backyard Worlds: Planet 9 website allows the general public to inspect videos combining images from the sky through the years in order to identify celestial objects that move over time. The goal: look for the hypothetical “Planet 9” ‒ whose existence was conjectured in 2019 by two astrophysicists ‒ and identify brown dwarfs in our neighborhood of the galaxy.
The two brown dwarfs caught the astrophysicists’ attention because of their very strange properties: they’re thought to be very old and are possibly part of the first generations of stars and brown dwarfs formed in the disk of our galaxy, about 10 billion years ago. At that time, interstellar gas was composed almost exclusively of hydrogen and helium, with no heavier elements. For that reason, the composition of these two brown dwarfs is highly atypical. We think that they’re just about 75 times more massive than Jupiter, almost massive enough to have become stars. Some researchers even believe that nuclear reactions might have started, then stopped a number of times early in their lives, a bit like a car engine that stalls! Future study of these new brown dwarfs may well allow us to better understand the chemical conditions in our galaxy’s distant past, and to learn more about the nuclear reactions that take place at the core of small stars.
A video showing the movement of WISEA J181006.18−101000.5 (red dot at the center) compared to the background stars between 2010 and 2019, as seen by the WISE telescope. Credit: Dan Caselden, WiseView.