Focus on Do Earth-like planets orbit our galaxy's black hole?
‘Blanets’ could form from the ice-covered dust particles that circle these cosmic colossi
Supermassive black holes (SMBHs) are theorised to be present at the centres of galaxies across the universe. They are Herculean celestial objects that consume gas and dust via an accretion disc and burp out jets of X-ray radiation from their core – jets so powerful they can be spotted from billions of light years away. This doesn’t sound like a hospitable environment for planets, but recent research led by Dr Keiichi Wada, an astrophysicist at Kagoshima University in Japan, suggests it could be.
Wada and his research team believe that there could be tens of thousands of these black hole planets, or ‘blanets’, at the heart of the Milky Way. The formation of a planet is a complicated transition from small clumps of debris to enormous orbs of rock, gas and liquid. It’s an even trickier process around a SMBH since not all of these circumnuclear discs can accommodate planet formation. These regions are far less dense than what is found around young stars, and the emissions near a black hole’s event horizon can prevent gas and ice from accreting effectively.
Ice is an essential part of this formation process; it allows for debris to coalesce before the mass latches onto the sticky frozen surface. Some SMBHs only have ice orbiting them at a certain distance from their centre, where it is cool enough, known as the ‘snow line’. Wada theorises that blanets can form beyond the snow line and form rocky planets – like Earth, except as much as ten-times larger – and gas giant planets similar to Neptune. These rocky blanets would take around 10 million years to come to fruition, and if conditions are right they can accrete even more debris, enabling them to evolve into gas giants.
The formation of blanets can only happen around a percentage of SMBHs, however. If a black hole is extremely powerful and active, it will have an increased radiation output that will melt ice and make it impossible for a significant amount of accretion. Think of it as throwing two pebbles together and trying to make them stick, as opposed to throwing two snowballs together.
Sagittarius A* (Sgr A*), the black hole at the core of the Milky Way, is cool enough to allow for ice in its circumnuclear disc. According to Wada’s calculations, thousands of blanets are likely to have been born around it – the problem, however, is that it’s extremely unlikely that we will ever observe one, since they would be hidden from sight, buried in a thick swathe of matter.
“Wada theorises that blanets can form beyond the snow line and form rocky planets”