There are now five different lines of observational evidence pointing to the existence of Planet Nine, a super-Earth-sized world about 20 times farther from the Sun than Neptune, according to Caltech planetary astrophysicist Konstantin Batygin and co-authors.
Artist’s impression of Planet Nine. Image credit: Tom Ruen / ESO.
Dr. Batygin and Caltech astronomer Mike Brown described the first three breadcrumbs on Planet Nine’s trail in a January 2016 paper, published in the Astronomical Journal.
Six known objects in the distant Kuiper Belt, a region of icy bodies stretching from Neptune outward toward interstellar space, all have elliptical orbits pointing in the same direction. That would be unlikely — and suspicious — enough.
But these orbits also are tilted the same way, about 30 degrees ‘downward’ compared to the pancake-like plane within which the planets orbit the Sun.
Computer simulations of the Solar System with Planet Nine included show there should be more objects tilted with respect to the solar plane. In fact, the tilt would be on the order of 90 degrees, as if the plane of the Solar System and these objects formed an ‘X’ when viewed edge-on.
Sure enough, Dr. Brown realized that five such objects already known to astronomers fill the bill.
A second paper from the team, this time led by Dr. Batygin’s graduate student, Elizabeth Bailey, showed that Planet Nine could have tilted the planets of our Solar System during the last 4.5 billion years.
This could explain a longstanding mystery: why is the plane in which the planets orbit tilted about 6 degrees compared to the Sun’s equator?
“Over long periods of time, Planet Nine will make the entire solar-system plane precess or wobble, just like a top on a table,” Dr. Batygin said.
The last telltale sign of Planet Nine’s presence involves the Solar System’s contrarians: objects from the Kuiper Belt that orbit in the opposite direction from everything else in the Solar System.
Planet Nine’s orbital influence would explain why these bodies from the distant Kuiper Belt end up ‘polluting’ the inner Kuiper Belt.
“No other model can explain the weirdness of these high-inclination orbits. It turns out that Planet Nine provides a natural avenue for their generation. These things have been twisted out of the Solar System plane with help from Planet Nine and then scattered inward by Neptune,” Dr. Batygin said.
The remaining step is to find Planet Nine itself. Dr. Batygin and Dr. Brown are using the Subaru Telescope at Mauna Kea Observatory in Hawaii to try to do just that.
But where did Planet Nine come from? “I spend little time ruminating on its origin — whether it is a fugitive from our own Solar System or, just maybe, a wandering rogue planet captured by the Sun’s gravity,” Dr. Batygin said.
“I think Planet Nine’s detection will tell us something about its origin.”
Other scientists offer a different possible explanation for the Planet Nine evidence cited by Dr. Batygin.
A recent analysis based on a sky mapping project called the Outer Solar System Origins Survey, which discovered more than 800 new ‘trans-Neptunian objects,’ suggests that the evidence also could be consistent with a random distribution of such objects.
Still, the analysis, from a team led by University of Victoria researcher Cory Shankman, could not rule out Planet Nine.
If Planet Nine is found, it will be a homecoming of sorts, or at least a family reunion.
Over the past 20 years, surveys of extrasolar planets in our Milky Way Galaxy have found the most common types to be ‘super-Earths’ and their somewhat larger cousins — mini-Neptunes.
Yet these common, garden-variety planets are conspicuously absent from our Solar System.
Weighing in at roughly 10 times Earth’s mass, the hypothetical Planet Nine would make a good fit. Planet Nine could turn out to be our missing super-Earth.
Dr. Batygin’s latest paper, co-authored with Alessandro Morbidelli of the Observatoire de la Côte d’Azur, will appear in the Astronomical Journal and is now available online at arXiv.org.
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Konstantin Batygin & Alessandro Morbidelli. 2017. Dynamical Evolution Induced by Planet Nine. AJ, in press; arXiv: 1710.01804
This article is based on text provided by the National Aeronautics and Space Administration.
