Astronomers have discovered a new dwarf planet in the Oort Cloud, a spherical ‘cloud’ of frozen objects surrounding the Solar System and extending halfway to the nearest stars. Named 2015 TG387, the new object has an orbit that supports the presence of an unknown massive distant planet, called Planet Nine.
This image of 2015 TG387 was taken by the Subaru Telescope on October 13, 2015. Image credit: Dave Tholen / Chad Trujillo / Scott Sheppard.
Nicknamed ‘the Goblin,’ 2015 TG387 has a diameter of about 186 miles (300 km).
The dwarf planet was discovered about 80 AU (astronomical units) from the Sun, a measurement defined as the distance between the Earth and Sun. For context, Pluto is around 34 AU, so the object is about 2.5 times further away from the Sun than Pluto is right now.
2015 TG387 is on a very elongated orbit and never comes closer to the Sun, a point called perihelion, than about 65 AU.
Only two extreme objects 2012 VP113 and Sedna at 80 and 76 AU respectively have more-distant perihelia than 2015 TG387.
Though 2015 TG387 has the third-most-distant perihelion, its orbital semi-major axis is larger than 2012 VP113 and Sedna’s, meaning it travels much farther from the Sun than they do. At its furthest point, it reaches all the way out to about 2,300 AU.
2015 TG387 is one of the few known objects that never comes close enough to the Solar System’s giant planets, like Neptune and Jupiter, to have significant gravitational interactions with them.
“These so-called Inner Oort Cloud objects like 2015 TG387, 2012 VP113, and Sedna are isolated from most of the Solar System’s known mass, which makes them immensely interesting,” said Dr. Scott Sheppard, an astronomer at the Carnegie Institution for Science.
“They can be used as probes to understand what is happening at the edge of our Solar System.”
“We think there could be thousands of small bodies like 2015 TG387 out on the Solar System’s fringes, but their distance makes finding them very difficult,” added Dr. David Tholen, an astronomer at the University of Hawaii.
The orbits of 2015 TG387 and its fellow Inner Oort Cloud objects 2012 VP113 and Sedna, as compared with the rest of the Solar System. Image credit: Roberto Molar Candanosa & Scott Sheppard, Carnegie Institution for Science.
2015 TG387 was discovered as part of the team’s ongoing hunt for unknown dwarf planets and Planet Nine.
“These distant objects are like breadcrumbs leading us to Planet Nine,” Dr. Sheppard said.
“The more of them we can find, the better we can understand the outer Solar System and the possible planet that we think is shaping their orbits — a discovery that would redefine our knowledge of the Solar System’s evolution.”
The location in the sky where 2015 TG387 reaches perihelion is similar to 2012 VP113, Sedna, and most other known extremely distant trans-Neptunian objects, suggesting that something is pushing them into similar types of orbits.
The astronomers ran computer simulations for how different hypothetical Planet Nine orbits would affect the orbit of 2015 TG387.
The simulations included a super-Earth-mass planet at several hundred AU on an elongated orbit as proposed by Caltech researchers Konstantin Batygin and Michael Brown in 2016.
Most of the simulations showed that not only was 2015 TG387’s orbit stable for the age of the Solar System, but it was actually shepherded by Planet Nine’s gravity, which keeps the smaller 2015 TG387 away from the massive planet.
This gravitational shepherding could explain why the most-distant objects in our Solar System have similar orbits. These orbits keep them from ever approaching the proposed planet too closely, which is similar to how Pluto never gets too close to Neptune even though their orbits cross.
“What makes this result really interesting is that Planet Nine seems to affect 2015 TG387 the same way as all the other extremely distant Solar System objects,” said Dr. Chad Trujillo, an astronomer at Northern Arizona University.
“These simulations do not prove that there’s another massive planet in our Solar System, but they are further evidence that something big could be out there.”
A paper with the full details of the discovery will be published in the Astronomical Journal.
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Scott Sheppard et al. 2018. A New High Perihelion Inner Oort Cloud Object. AJ, in press; arXiv: 1810.00013
