14 | New Scientist | 28 September 2019
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ultra-fast EV charging%3LVGHYHORSLQJDGYDQFHGIXHOVWRKHOSFDUVUXQPRUHHĮFLHQWO\WE ARE used to thinking of
planets orbiting stars, but it turns
out they might exist around
supermassive black holes too.
Scientists have floated the idea
of planets orbiting smaller black
holes before. But far less is known
about whether this would be
possible for supermassive black
holes, which are millions of times
more massive than our sun.
Keiichi Wada at Kagoshima
University in Japan and his
colleagues applied models of
planet formation to these black
holes to see what would happen.
“This is the very first study that
claims a possibility of ‘direct’
formation of planet-like objectsthat are not associated with stars,
but with supermassive black
holes,” says Wada.
Planet formation is generally
thought to begin with a disc
of dust and gas around a star.
Gradually, this material clumps
together and its gravity draws in
more material, building a planet.
Wada and his team examined
how similar discs known to
surround supermassive black
holes would behave (arxiv.org/
abs/1909.06748). They showed
that roughly the same process
could operate. “Basically it is the
same as the formation of normal
planets around stars,” says Wada.
Because of their huge mass andgravitational pull, supermassive
black holes can warp space-time in
strange ways, creating effects such
as time dilation. Orbiting planets
might not feel these effects,
however. Wada says they would
probably orbit at a huge distanceof between 10 and 30 light years
from the black hole, where the
extreme effects of general
relativity would be “negligible”.
Planetary systems around a
supermassive black hole probablywouldn’t be just like those around
star systems, though. “The total
amount of dust is enormous,” says
Wada. This means the typical mass
of planets would probably be large,
about 10 times more massive than
Earth, and there could be as many
as 10,000 of them around a single
black hole, says Wada.
Sean Raymond at the
Laboratory of Astrophysics of
Bordeaux, France, says the team’s
logic is plausible, but that it might
be possible for planets to form
closer in. It is also “theoretically
possible for millions of planets
to orbit a supermassive black
hole, but it requires a lot of things
to be just perfect,” he says. ❚AstrophysicsJonathan O’CallaghanPlanets could be orbiting black holes
Such worlds would be huge and there could be millions of them
10,
Number of planets that could orbit
a single supermassive black hole