BBC Science Focus

Planet Nine could be a grapefruit-sized black hole, say astrophysi­cists

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Astrophysi­cists have suggested that the hypothetic­al Planet Nine could be a tiny black hole, and they’ve proposed a way to find out if this is really the case.

Planet Nine has never been seen directly, but the existence of a ninth planet orbiting our Sun could explain certain features of the outer Solar System, such as the clustering together of icy rocks called ‘trans-Neptunian objects’ with similarly tilted orbits.

Last year, scientists in the UK and US suggested that Planet Nine could instead be a primordial black hole. These as-yet-unseen black holes are thought to have formed a fraction of a second after the Big Bang. If Planet Nine was such an entity, it would be about the size of a grapefruit, with a mass of five to ten times that of 'arth.

Now, two astrophysi­cists at Harvard University, US, have suggested a way to investigat­e this: look for the bursts of electromag­netic radiation, or ‘accretion flares’, that would be produced as this black hole ripped apart and swallowed any nearby icy objects.

The authors propose using the upcoming Legacy Survey of Space

“We do not know exactly where to look for Planet Nine”

and Time (LSST) to look for the accretion flares. The LSST will use a 3.2-gigapixel camera at Chile’s Rubin Observator­y to photograph the entire visible sky every few nights for a period of 10 years.

“LSST has a wide field of view, covering the entire sky again and again, and searching for transient flares,” said study co-author Prof Avi Loeb. “Other telescopes are good at pointing at a known target, but we do not know exactly where to look for Planet Nine. We only know the broad region in which it may reside.” “The outskirts of the Solar System is our backyard,” Loeb added.

“Finding Planet Nine [would be] like discoverin­g a cousin living in the shed behind your home [who] you never knew about.”

 ??  ?? Artist’s impression of accretion flares being produced as an icy comet is swallowed by a black hole
Artist’s impression of accretion flares being produced as an icy comet is swallowed by a black hole

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