Astronomers are still on the hunt for the elusive “Planet 9”

Astronomer Michael Brown led the campaign that controversially demoted Pluto in 2006 from the ninth planet in our solar system to just one of its many dwarf planets. Now he hopes to fill the void he has created with what he predicts will be the discovery of a “Planet 9” – a planet several times the size of Earth that could orbit the sun far beyond Neptune. .

“That was certainly not the intention,” said Brown, professor of planetary astronomy at the California Institute of Technology in Pasadena and author of the memoir “How I Killed Pluto and Why He Bred It In.” “.

“If I was prescient enough to have had all of these ideas ahead of time and then demote Pluto and find a new Planet 9, then that would be great – but it’s really just a coincidence.”

A study published online in August by Brown and his colleague at Caltech, astrophysicist Konstantin Batygin, re-examines the evidence for a proposition they first suggested in 2016 – that the hypothetical Planet 9 could explain the anomalies observed by astronomers in the Outer Solar System, in particular the unusual clustering of icy asteroids and cometary nuclei known as Kuiper Belt objects. The study has been accepted for publication by the Astronomical Journal, according to National Geographic.

Despite years of research, Planet 9 has never been seen. As a result, some astronomers have suggested that it does not exist and that the clustering of objects noted by Brown and Batygin is the result of “observation bias” – since less than a dozen objects have been seen, their regrouping could be a statistical chance which would not be seen among the hundreds which one thinks to exist.

For their latest study, however, Brown and Batygin added several recent sightings of objects, and they calculated that the clustering is almost certainly real.

That would suggest Planet 9 is almost certainly there – and the new study includes a “treasure map” of its supposed orbit that shows astronomers the best places in the sky to look for it.

Brown is working with data from several astronomical surveys, hoping to get a first glimpse of Planet 9. If that research fails, he hopes it can be seen in survey data from a new large telescope at the Vera Rubin Observatory. in the mountains of northern Chile, which is expected to start fully functioning in 2023.

One of the results of the new study is that Planet 9’s orbit is closer to the sun than the 2016 study proposed, with an elongated orbit only about 380 times the distance between Earth and the Sun at closer, instead of over 400 times that distance.

The closer orbit would make Planet 9 much brighter and much easier to see, Brown said, although their recalculations suggest it is also a bit smaller – about six times the mass of Earth, instead of 20 times larger.

“Being closer, even though it’s a little less massive, it’s a little brighter than we originally expected,” he said. “So I’m delighted that this helps us find him much faster. “

If Planet 9 exists, it’s probably a very cold gas giant like Neptune, rather than a rocky planet like Earth. It would be smaller, however – Neptune is over 17 times the mass of Earth. But about six to ten times the mass of Earth is the most common size of gas giants observed by astronomers elsewhere in our galaxy, although there are none – so far – in our system. solar, said Brown.

While planet 9 could have formed such a great distance from the gas disk around the early sun, it seems likely that it formed at about the same distance from the sun as Uranus and Neptune, but it was thrown into the confines of the Sun. system by the strong gravity of Saturn, he said.

He rejected the suggestion made by astronomers last year that Planet 9 could in fact be a black hole orbiting the sun. “It was almost a joke when they wrote this article,” he said. “It’s funny and it’s cute – but there’s really no reason to speculate that it could be a black hole.”

As Brown and his colleagues renew their search for Planet 9 with a better idea of ​​where to look, some other astronomers remain skeptical of its existence.

Physicist Kevin Napier, a graduate student at the University of Michigan at Ann Arbor, led a study published earlier this year that suggested that the clustering of objects in the Kuiper Belt was a statistical illusion.

He said in an email that the extremely low number of object orbits used as evidence for the existence of Planet 9 – only 11 are known – is not convincing.

“There is only a limited amount of statistical power that can be derived from a dozen data points,” he said.

This means that the existence of Planet 9 can only be conjectured until more observations are made of the outer solar system.

“Maybe we’ll discover a new planet lurking in darkness, or maybe our findings will make all evidence of clustering disappear altogether,” he said. “Until then, we will continue to search the skies for new and interesting rocks, and in doing so, we will more clearly focus our understanding of our solar system. “