Another Dwarf Planet in Our Solar System?

From :- Sky & Telescope  

By :- David L. Chandler

Edited by :- Amal Udawatta

Image showing the current location of Pluto, Neptune, and 2017 OF201.
Jiaxuan Li and Sihao Cheng

A newly discovered object in the outer solar system, 2017 OF201, is the largest found in more than a decade.

It was hiding in plain sight, lurking deep inside terabytes of publicly available data, some of which are more than a decade old. But this particular needle in a haystack — the first new dwarf planet in the outer solar system to be found in more than a decade — took months of computational work to ferret out from the mass of background stars and noise.

The newfound object, which for now bears the unwieldy name of 2017 OF201, is approximately 700 kilometers (400 miles) wide and follows an extremely elliptical orbit around the Sun that takes an estimated 25,000 years to complete. Its size puts it in the category of dwarf planets, along with Pluto, the asteroid Ceres, and other objects. It’s one of only a half-dozen or so (depending on exactly which definitions are used) dwarf planets now known in the outer solar system.

The discovery, first reported last week, was the result of painstaking analysis of data collected by the Dark Energy Camera. A team of astronomers conducting a survey with that camera is primarily looking for the effects of gravitational lensing in distant galaxies.  But as the publicly available data are sensitive to extremely faint objects, they’re also “very suitable” for searching for distant objects in the solar system, says Sihao Cheng (Institute for Advanced Study), who led the discovery team.

In principle, anyone could have made this discovery. Cheng points out that he and co-discoverer Jiaxuan Li have been amateur astronomers (and Sky & Telescope readers) since they were in elementary school. But finding the elusive tracks of a faint, moving point of light against a rich background of stars in images that span years of intermittent observation “requires a very good algorithm and a lot of computation and a lot of time,” Cheng says. Just developing the algorithm took several months, he says, followed by additional months of computation as they scanned through roughly 200 terabytes of data using hundreds of processors.

After the new object showed up in DES data, Cheng and his team computed its orbit and determined that it should also show up in another public dataset available from the Canada France Hawaii telescope. Sure enough, they found another nine images showing the object in exactly the predicted positions, bringing the total span to 19 observations going from 2011 to 2017.

“This is the frontier of discovery,” Cheng says. “These datasets are a bit difficult to search, so we are the first ones to do it.”

Composite image showing the five dwarf planets recognized by the International Astronomical Union, plus the newly discovered trans-Neptunian object 2017 OF201.
Images of dwarf planets: NASA/JPL-Caltech; image of 2017 OF201: Sihao Cheng et al.

One thing that made the search tricky, he tells Sky & Telescope, is the widely separated observation dates. Whereas a targeted search for asteroids would typically take successive exposures separated by hours or days, these images were often separated by months or even years. They therefore contained a lot of points of light from unrelated objects, such as asteroids. “You need to find a way to eliminate these contaminations efficiently,” Cheng adds.

The object is also only visible over a small fraction of its orbit. Its extremely elongated track around the Sun makes it too faint to see for more than 99% of its orbit.

“The fact that we observed this one single object implies that there could exist a hundred times more objects like this at the edge of our solar system,” says team member and graduate student Eritas Yang (Princeton University). But they will be hard to find. Even with the significantly deeper observations soon to be made by the new Vera Rubin Observatory in Chile, at best only one or two more such objects might turn up.

The team hopes to carry out followup observations to learn about the details of this dwarf planet’s orbit, size, and composition, possibly using the Hubble Space Telescope and the James Webb Space Telescope as well as ground-based telescopes. Deeper observations may even reveal a moon, as is common among outer solar system objects.

Images of 2017 OF201 from the telescope database and its trajectory in the sky.
Jiaxuan Li and Sihao Cheng

What About Planet X?

The orbits of some other outer solar system objects, specifically those whose orbits are larger than that of Neptune (known as trans-Neptunian objects, or TNOs) appear to cluster, leading some astronomers to suggest that they’re feeling the gravitational influence of an unknown far-out major planet, dubbed Planet X or Planet 9. But this new dwarf planet discovery doesn’t seem to fit that pattern. Cheng and his team say this puts the Planet X hypothesis into question, since if that planet existed, 2017 OF201’s orbit would be unstable.

But Konstantin Batygin (Caltech), a coauthor of the Planet X hypothesis, disagrees with that conclusion. “[The simulations show] this object is strongly interacting with Neptune,” he says.

“The object is unstable, so it means virtually nothing for the Planet 9 hypothesis.” Cheng, however, says that the object is right at the boundary between being stable and unstable.

Although he’s skeptical of their claims regarding Planet X, Batygin has high praise for the team’s discovery. “The authors did a heroic effort of combing through the data and finding this body,” he says. “It’s a very sophisticated exercise.”