New dwarf planet nicknamed The Goblin

Artist’s concept of the newly discovered dwarf planet, nicknamed The Goblin (aka 2015 TG387). Image via Roberto Molar Candanosa/Scott Sheppard/Carnegie.

Astronomers have found a goblin in the far reaches of the outer solar system. No, it’s not a mythical space-faring creature. The Goblin is the nickname given to a new dwarf planet in the outer solar system. It’s an exciting discovery on its own, and the orbit of The Goblin also supports the possible existence of the long-sought-after – and much larger – Planet X.

Scott Sheppard at Carnegie Institution for Science, Chad Trujillo at Northern Arizona University and David Tholen at the University of Hawaii announced the finding on October 2, 2018 via an electronic circular from the IAU’s Minor Planet Center. A paper has been submitted to the The Astronomical Journal. In other words, astronomers just found this object.

They found it during an intensive search for Planet X, a search these astronomers said was:

… the deepest and most comprehensive survey of its kind so far.

The orbit of the new dwarf planet places it in a group of small, distant worlds that astronomers call Inner Oort Cloud objects. In other words, it resides in the Oort Cloud – a giant spherical shell of icy objects surrounding our sun, thought of as the realm of comets, far beyond the orbits of Pluto and less distant Kuiper Belt Objects.

The Goblin, also known as 2015 TG387, is about 80 astronomical units (AU) from the sun, with one AU defined as the distance between the sun and Earth. By comparison, Pluto is 34 AU from the sun, so The Goblin is about two and a half times farther away from the sun as Pluto, right now.

It’s thought to be quite small, about 200 miles (300 km) in diameter. Its orbit is very elongated, never bringing the object closer to the sun than 65 AU. Besides 2015 TG387, the only other objects with more-distant perihelia – that is, more distant nearest-the-sun points – are 2012 VP113 and Sedna, at 80 and 76 AU respectively.

However, the orbital semi-major axis of The Goblin is larger than that of both 2012 VP113 and Sedna, meaning it travels much farther from the sun along its orbit than they do. At its most distant point, it is an incredible 2,300 AU from the sun.

The orbit of The Goblin compared to those of some other Inner Oort Cloud objects. Image via Roberto Molar Candanosa/Scott Sheppard/Carnegie Institution for Science.

As Scott Sheppard explained:

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. They can be used as probes to understand what is happening at the edge of our solar system.

Sheppard and Trujillo also discovered 2012 VP113, in 2014. Finding such objects means that there are probably many more yet to be seen. According to David Tholen:

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. Currently we would only detect 2015 TG387 when it is near its closest approach to the sun. For some 99 percent of its 40,000-year orbit, it would be too faint to see.

Graph showing the distances of the planets and Kuiper Belt compared to The Goblin (not to scale). Image via Roberto Molar Candanosa/Scott Sheppard/Carnegie Institution for Science.

But it’s not only those objects themselves that are of interest to astronomers. These very distant objects might also be pointing the way to finally locating another major planet in our solar system. As noted by Sheppard:

These distant objects are like breadcrumbs leading us to Planet X. 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.

One major clue that The Goblin may be evidence for the larger Planet X is that the location in the sky where 2015 TG387 reaches perihelion is similar to that of 2012 VP113, Sedna, and most other known extremely distant trans-Neptunian objects; this could be explained by something “pushing” them into similar types of orbits. To test this further, Chad Trujillo and Nathan Kaib at the University of Oklahoma ran computer simulations to see how various hypothetical Planet X orbits would affect the orbit of 2015 TG387. Since most previous estimates of Planet X’s size say it is significantly larger than Earth, the simulations included a super-Earth-mass planet at several hundred AU on an elongated orbit, as had been proposed by Caltech’s Konstantin Batygin and Michael Brown in 2016.

Discovery images of The Goblin (2015 TG387) taken by the Subaru 8-meter telescope on Mauna Kea in Hawaii on October 13, 2015. The two images were taken about 3 hours apart. Image via Scott Sheppard.

So what was the result? Most of the simulations suggested that not only was 2015 TG387’s orbit stable for the entire age of the solar system, but it was actually shepherded by Planet X’s gravity – that would keep the much smaller 2015 TG387 away from the much more massive Planet X. Such gravitational shepherding – similar to how Saturn’s smallest moons “shepherd” its rings – could also explain why these distant objects in the solar system all have similar orbits. According to Trujillo:

What makes this result really interesting is that Planet X seems to affect 2015 TG387 the same way as all the other extremely distant solar system objects. 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.

If Planet X is confirmed to exist, it may indeed turn out to be – as suggested by some scientists – a super-Earth, a rocky planet larger than Earth but smaller than Uranus or Neptune. Many such worlds have already been found in other solar systems, so discovering a previously unknown one here as well would be … well, very exciting!

Bottom line: While the elusive Planet X hasn’t been found yet, the discovery of The Goblin is intriguing, and provides another clue that Planet X should be out there, waiting to be detected by earthly telescopes.

Via Carnegie Institution for Science