Expansive Sky Survey Finds No Sign of Planet 9

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In just a few years, there will be legal adults who never lived in a solar system with nine official planets. There have been eight since the 2006 demotion of Pluto, but several years ago, Astronomers Mike Brown and Konstantin Batygin theorized there might be a ninth undiscovered planet at the edge of deep space. It’s proven difficult to confirm, though. A recent survey using the powerful Atacama Cosmology Telescope (ACT) in Chile has come up empty despite scanning a huge swath of the sky, casting even more doubt on Planet 9’s existence. 

All the evidence we currently have for Planet 9 is circumstantial. As observational technology has improved, astronomers have been able to map the orbits of small trans-neptunian objects (TNOs), which zip around in the vast emptiness at the edge of the solar system. Brown and Batygin noted an unusual clustering of orbits among TNOs that could be explained by a previously unknown massive object out there. 

There have been several surveys attempting to find Planet 9, but so far none of them have spotted anything. The hypothesized world would be five to ten times the size of Earth, but even something that large is a mere speck from this far away. The team that conducted the latest ACT scan acknowledges how difficult it will be to find such a planet, but the ACT is a good way to go looking. The 6-meter Atacama Cosmology Telescope is intended to study cosmic background radiation in millimeter wave frequencies, but its high resolution and sensitivity make it capable of finding dim objects like phantom planets. 

The Atacama Cosmology Telescope as seen from above.

Over the past six years, ACT has scanned 87 percent of the southern hemisphere’s sky. At an estimated 400 to 800 times the distance between the sun and Earth, Planet Nine would be completely shrouded in darkness. The team processed those images to search for fainter mmWave sources that could end up being the fabled Planet 9. However, the team says they have 95 percent confidence Planet 9 did not appear in any of the images. 

So is hope for Planet 9’s existence dead? Not yet. We may simply lack the necessary technology to find it right now, or it could be in an area where ACT can’t see it. As more powerful telescopes like the Vera C. Rubin Observatory come online, we could have the tools necessary to improve our picture of the outer solar system. That said, there are other potential explanations for the odd orbits of TNOs, for example clusters of TNOs could themselves be the cause. Until someone can prove Planet 9 is out there, we’re sitting at eight planets. Well, probably.

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