Planet hunters are on the search for exoplanets, or planets outside of our own Solar System. But beyond that they pushing the boundary of analysing the spectra of atmospheres of exoplanets. The direct detection of a new brown dwarf around the star HD13724 from a team led by astrophysicist Emily Rickman provides an important analogue towards understanding exoplanetary atmospheres.
Brown dwarfs are an intermediate mass object between a giant planet and a low mass star. They are easier to detect directly than exoplanets because they are much more bright and massive than a planet, especially when they are young and still hot from forming. A direct detection is quite literally taking a direct image of a companion orbiting a star.
When these objects are observed, large telescopes are required like the ‘Very Large Telescope (VLT)’ based at Paranal Observatory, Chile. Rickman et al. 2020 use the instrument SPHERE on the VLT to accomplish this detection. SPHERE has a coronograph which acts as a mask to block out as much starlight as possible in order to see a much fainter companion next to it. In this image on the left the star is masked by a white circle, where the coronograph is present in the image. To the right a bright blob is seen and that is the direct detection of a companion orbiting another star.
The brown dwarf is estimated to be around 26 astronomical units (AU) away from its star, which is 26 times the distance between Earth and the Sun, and have a mass that is 50 times that of Jupiter. The further away and more massive a companion is from its star (and therefore away from the bright starlight), the easier it becomes to directly image a planet or brown dwarf.
The SPHERE instrument on the VLT also has capabilities of measuring the spectrum of other worlds. In the image on the right, the black points represent measurements of the flux of the brown dwarf taken using SPHERE across a range of wavelengths. The dips and peaks in spectrum represent different molecules like methane and water.
The discovery of such a companion by Rickman et al. is crucial in calibrating atmospheric models of planets around other stars, meaning it will act as a benchmark to calibrate observations of real objects with theories to explain what planet hunters are seeing. As instrumentation improves and telescopes get bigger, the possibility of detecting smaller and smaller objects will become possible. Therefore, it is necessary to test atmospheric models on objects like these to help further the understanding of exoplanetary atmospheres.
Reference: Rickman et al. 2020, Spectral and atmospheric characterisation of new benchmark brown dwarf HD13724B