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.
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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.
References:
- 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]