December 14, 2016 – Astrophysicist Jeff Linsky and his colleagues recently created a sophisticated mathematical model of the outer atmosphere of the small M-dwarf star called GJ832. The new model fits well with spectral observations of the star made with the Hubble Space Telescope (HST).
This accomplishment bodes well for two reasons: First, it provides a tool for better understanding M-dwarf stars––the most common type of star in our galaxy. Second, it may aid in determining whether oxygen in the atmospheres of planets orbiting M-dwarf stars is produced by photochemistry or by living organisms such as green plants on Earth.
Astronomers have already discovered several thousand planets orbiting other stars, including two around GJ832, which is 16.1 light years away from Earth and one around Proxima Centauri, just 4.25 light years away. The planet orbiting Proxima Centauri, Proxima b, is just a little larger than Earth, and it orbits inside the “habitable” zone where liquid water could exist.
“Since the nearest star to the Sun has a rocky planet inside the habitable zone, then there’s going to be an enormous number of rocky planets in the habitable zones around other stars,” Linsky said.
Most stars in the galaxy are smaller, cooler M-dwarfs stars (including GJ832 and Proxima Centauri). Thus, it’s likely that the first habitable planets discovered in the next few years orbit M-dwarfs. For this reason, Linsky and his colleagues wanted to understand the spectra emitted by a typical M-dwarf star because a star’s light affects the atmospheres of the planets orbiting it. The researchers chose GJ832 because it is relatively close to Earth, and Hubble had already acquired excellent observations of its spectral characteristics.
What Linsky and his colleagues did was come up with a complete theoretical description of GJ832’s upper atmosphere, or chromosphere. The chromosphere is the origin of the radiation from the star that has been observed by the HST. The new model is so good that it can now be used to accurately predict the spectrum of an M-dwarf that cannot be directly observed.