Hypervelocity Planets

At the centre of the Milky Way sits a supermassive black hole (SMBH) with a mass of around 4 million times the mass of the Sun. A close encounter of a binary star system with the SMBH can cause one star to be ejected as a hypervelocity star (HVS) with sufficient velocity to escape the gravitational pull of the Milky Way, while the other star becomes captured into an eccentric orbit around the SMBH. This is the most likely mechanism for the production of HVSs. The existence of HVSs was first theorised in 1988 and the first HVS was discovered in 2005. Several HVSs have been discovered since. Ordinary stars in the galaxy have velocities on the order of ~100 km/s, while HVSs have velocities on the order of ~1000 km/s.

A study by Ginsburg, Loeb & Wegner (2012) investigates what happens when a binary star system hosting a planetary system gets disrupted by the SMBH at the centre of the Milky Way. In particular, the study examines the generation of hypervelocity planets (HVPs). The possible outcomes from such an interaction are - a HVS, one or more HVPs, a HVS with one or more bound planets, a star left behind in an orbit around the SMBH with one or more bound planets, a planet collides into its host star, or one or more planets left behind in independent orbits around the SMBH (Figure 2 & 3).

Figure 1: Artist’s impression of a lone planet in the outskirts of a galaxy.

Figure 2: The panel illustrates the possible outcomes after a binary star system with 4 planets is disrupted by the SMBH at the Milky Way’s centre. After binary disruption a HVS is produced with two bound planets. The second star remains in a highly eccentric orbit around the SMBH. The second star’s planets are removed, and the first planet falls into a highly eccentric orbit close to the SMBH, while the second planet is ejected into a much larger, but also highly eccentric orbit around the SMBH. Ginsburg, Loeb & Wegner (2012).

Figure 3: The panel illustrates the possible outcomes after a binary star system with 4 planets is disrupted by the SMBH at the Milky Way’s centre. After binary disruption a HVS is produced with two bound planets. The second star remains in a highly eccentric orbit around the SMBH. The second star’s planets are both ejected as HVPs. Ginsburg, Loeb & Wegner (2012).

The study draws on a few sets of simulations. One particular set of simulations involves binary star systems with two planets (i.e. one planet per star). 1000 simulation runs were performed in this set of simulations. The mass of each star is set at 3 times the Sun’s mass, comparable to the masses of the presently known HVSs. The initial separation between both stars in the binary star system is 0.2 AU, while the star-planet separation varies with uniform probability in the range 0.02 to 0.04 AU. The simulation runs show that the average HVS velocity is ~1500 km/s, while the average HVP velocity is ~3000 km/s (Figure 4). The velocities of HVPs are on average ~1.5 to 4 times the velocities of HVSs. Another set of simulation runs involving binary star systems with four planets (i.e. two planets per star) produce qualitatively similar results.

For a binary star system with two planets, the probability of producing a HVP is ~30 to 40 percent. For a binary star system with four planets, the probability increases to ~70 to 80 percent. The velocity distribution of HVPs reveals a small number of HVPs with exceptionally high velocities of ~10,000 km/s (Figure 4). These extreme outliers speed through space at a few percent the speed of light. A HVP travelling at a speed of 10,000 km/s would traverse a distance of one light year in just 30 years. An observer on such a planet can see the constellations change in a matter of years while the planet is travelling through the galaxy. As the planet speeds out of the galaxy, the Milky Way would appear as a receding disk of light. Such a planet is destined to travel through the immense intergalactic void separating galaxies and clusters of galaxies.

Figure 4: Velocity distribution of HVSs and HVPs. This sample comes from 1000 simulation runs involving a binary star system with two planets. Ginsburg, Loeb & Wegner (2012).

“These warp-speed planets would be some of the fastest objects in our galaxy. If you lived on one of them, you’d be in for a wild ride from the centre of the galaxy to the universe at large,” said astrophysicist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics and co-author of the study. “Other than subatomic particles, I don’t know of anything leaving our galaxy as fast as these runaway planets,” added lead author Idan Ginsburg of Dartmouth College.

Reference:
Ginsburg, Loeb & Wegner (2012), “Hypervelocity Planets and Transits Around Hypervelocity Stars”, arXiv:1201.1446 [astro-ph.GA]