Sunday, August 10, 2014

Swarm of Planets Circling a Supermassive Black Hole

A supermassive black hole (SMBH) with ~4 million times the Sun’s mass sits in the centre of the Milky Way. Over the decades, observations have revealed the presence of a group of stars known as the S-stars that orbit very close to the SMBH. The presently known S-stars are stars that are more massive and more luminous than the Sun. This indicates a larger population of fainter members that continue to elude detection. The S-stars zip around the SMBH with speeds of up to ~10,000 km/s. Currently, S0-102 holds the record for being the star with the shortest known orbital period around the SMBH at the galaxy’s centre. S0-102 has an orbital period of 11.5 years.

The S-stars are believed to have originated from binary star systems that are disrupted due to close passages near the SMBH. When a binary star system is disrupted, one star can get ejected from the vicinity of the SMBH, while the other star is left behind in a tight orbit around the SMBH. Since planets are ubiquitous around stars, it is reasonable to assume that these disrupted binary star systems have planets of their own, at least before they became disrupted. One study by Ginsburg, Loeb & Wegner (2012) show that such disruption events can strip planets from their host stars. These planets are either left behind on independent orbits around the SMBH or ejected away from the SMBH as hypervelocity planets. Over time, planets that are left behind can create a swarm of planets around the SMBH.

Planets can attain terrific speeds if their orbits that take them sufficiently close to the SMBH. For example, a rocky planet with an Earth-like composition passing as close as it can to the SMBH without being tidally torn apart can reach up to several percent the speed of light. A head-on collision with even a small object at such a speed would be devastating for the planet. One study by Nayakshin, Sazonov & Sunyaev (2011) suggests that given the right conditions, a fragmentation cascade could destroy a swarm of planets around a SMBH. When an asteroid collides with a planet at very high velocity, it can shatter the planet, creating more fragments that can collide with more planets and so on. Such a process of fragmentation cascade could grind a swarm of planets around a SMBH into high velocity dust.

- Ginsburg, Loeb & Wegner (2012), “Hypervelocity Planets and Transits Around Hypervelocity Stars”, arXiv:1201.1446 [astro-ph.GA]
- Nayakshin, Sazonov & Sunyaev (2011), “Are SMBHs shrouded by "super-Oort" clouds of comets and asteroids?”, arXiv:1109.1217 [astro-ph.CO]