M-type (red dwarf) stars are cooler, low-mass, low-luminosity objects that make up the vast majority of stars in our Universe – accounting for 85% of stars in the Milky Way galaxy alone. In recent years, these stars have proven to be a treasure trove for exoplanet hunters, with multiple terrestrial (aka. Earth-like) planets confirmed around the Solar System’s nearest red dwarfs.
But what is even more surprising is the fact that some red dwarfs have been found to have planets that are comparable in size and mass to Jupiter orbiting them. A new study conducted by a team of researchers from the University of Central Lancashire (UCLan) has addressed the mystery of how this could be happening. In essence, their work shows that gas giants only take a few thousand years to form.
The study, which recently appeared in the journal Astronomy & Astrophysics, was the work of Dr. Anthony Mercer and Dr. Dimitris Stamatellos of the UCLan Jeremiah Horrocks the Institute for Mathematics, Physics & Astronomy (JHI – MPA). Dr. Mercer, an Astrophysics Reader with the JHI – MPA, led the research under the supervision of Dr. Stamatellos, who leads the institute’s “Theoretical Star formation & Exoplanets” group.
Together, they studied how planets could form around red dwarf stars to determine what mechanism would allow for the formation of super-massive gas giants. According to conventional models of planet formation, where the gradual build-up of dust particles leads to progressively bigger bodies, red dwarf systems should not have enough mass to form super-Jupiter-type planets.
To investigate this discrepancy, Mercer and Dr. Stamatellos used the UK Distributed Research using Advanced Computing (DiRAC) supercomputer – which connects facilities at Cambridge, Durham, Edinburgh, and Leicester University – to simulate the evolution of protoplanetary discs around red dwarf stars. These rotating discs of gas and dust are common around all newly borns stars and are what eventually lead to planet formation.
What they found was that if these young discs are large enough, they can fragment into different pieces, which would coalesce due to mutual gravitational attraction to form gas giant planets. However, this would require that the planets form within a few thousand years, a timescale that is extremely fast in astrophysical terms. As Dr. Mercer explained:
“The fact that planets may be able to form on such short timescale around tiny stars is incredibly exciting. Our work shows that planet formation is particularly robust: other worlds can form even around small stars in a variety of ways, and therefore planets may be more diverse than we previously thought.”
Their research also indicated that these planets would be extremely hot after they form, with temperatures reaching thousands of degrees in their cores. Because they don’t have an internal energy source, they would become fainter over time. This means that these planets would be easy to observe in the infrared wavelength when they are still young, but the window for direct observation would be small.
Still, such planets could still be observed indirectly based on their effect on their host star, which is how planets orbiting red dwarf stars have typically been found. This is known as the Radial Velocity Method (aka. Doppler Spectroscopy), where changes in the star’s spectra indicate that it is moving, which is an indication of planets exerting their gravitational influence on it. As Dr. Stamatellos added:
“This was the first time that we were able not only to see planets forming in computer simulations but also to determine their initial properties with great detail. It was fascinating to find that these planets are of the ‘fast and furious’ kind – they form quickly and they are unexpectedly hot.”
These results are nothing if not timely. Recently, astronomers detected a second extrasolar planet around Proxima Centauri, the closest star to our own. Unlike Proxima b, which is Earth-sized, rocky, and orbits within the star’s habitable zone; Proxima c is believed to be 1.5 times the size of Earth, half as massive as Neptune (making it a mini-Neptune), and orbits well-outside Proxima Centauri’s habitable zone.
Knowing that there is a possible mechanism that allows gas giants to form around red dwarfs stars puts us a step closer to understanding these entirely-common, but still-mysterious stars.