Astronomers have discovered an object — 2015 TG387 — that could help in the hunt for a hypothesized Planet Nine in the distant reaches of the solar system.

Orbits of 2015 TG<sub>387</sub>, Sedna, and 2014 VP<sub>113</sub
The object designated 2015 TG387, a.k.a. "The Goblin", orbits in the inner Oort Cloud along with Sedna and 2014 VP113.
Roberto Candanosa / Scott Sheppard / Carnegie Institution for Science

A newly found body in the extreme backwaters of the solar system incrementally bolsters the case for an undiscovered planet lurking far from the Sun.

Designated 2015 TG387 and nicknamed “The Goblin” by its discoverers, this object resides in the inner Oort Cloud, a region beyond the Kuiper Belt that until now harbored only two other known bodies: the dwarf planet Sedna and the less well-known 2012 VP113.

This far-flung body keeps its distance from the Sun, never coming closer than about 65 astronomical units (a.u.), or just under twice Pluto’s closest approach. However, that’s downright cozy compared to how far out it goes. A highly elongated orbit takes 2015 TG387 out to a whopping 2,300 a.u., much farther away than its two comrades in the inner Oort Cloud.

The team of Scott Sheppard (Carnegie Institution for Science), Chadwick Trujillo (Northern Arizona University), and David Tholen (University of Hawai'i) discovered 2015 TG387 in images acquired in mid-October 2015 at the 8.2-meter Subaru Telescope in Hawai‘i. At the time, it was about 80 a.u. from the Sun. Since then, the observers used other telescopes to track the object and refine its orbit. They estimate that the body is 300 km wide, roughly the width of Saturn’s moon Hyperion.

The International Astronomical Union’s Minor Planet Center announced this discovery on October 2nd. A detailed report has been submitted to the Astronomical Journal.

Animation of discovery images of 2015 TG387
Discovery images of 2015 TG387, taken 3 hours apart at the Subaru Telescope on October 13, 2015. 2015 TG387 is the dot moving near the center.
Scott Sheppard

Extrapolating from the three objects now known in the inner Oort Cloud, the team estimates that this largely unexplored region of space might harbor roughly 2 million bodies wider than 40 km. That puts the projected mass of this ensemble on par with that of the Kuiper Belt.

The bodies in the inner Oort Cloud, along with a smattering of other objects well beyond Neptune, are so remote that they are immune to the gravitational machinations of the known planets. That makes them unique probes for unknown forces at the limits of the solar system. For example, their aligned orbits suggest that a ninth planet, several times as massive as Earth, awaits discovery out there. While 2015 TG387 doesn’t make or break that case, the team’s simulations indicate that its orbit is at least consistent with a planet’s presence.


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bob kelly

October 3, 2018 at 6:54 am

Hi, Christopher, Thanks to S&T for getting an article about this object out so fast. After I hear about claims like these, I always check here to see what's up (no pun intended).
A couple of points - the date in the caption should be 2015? (Pointed out on facebook.)
The facebook post has an artist conception of an object. I've read the artwork is of the putative Planet X that may be showing its influence in organizing these newly found distant objects. Some may think it's a speculation on the newly discovered object in the article.
Feel free to read this and not post it or to take it down.

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Christopher Crockett

October 3, 2018 at 9:32 am

Thanks for the feedback, Bob! You mean they didn't discover the object in the future? We just like to stay well ahead of breaking news here. (Caption has been fixed.)

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October 5, 2018 at 4:32 pm

Is it my visual illusion or is there a smaller object moving in parallel with this object?

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October 5, 2018 at 8:51 pm

I am so disappointed---which, sadly, happens all too frequently—in the microscopic size of an image in this article. The expanded view of the orbit illustration is barely bigger than the thumbnail. A small thumbnail embedded in an article serves the excellent purpose of not taking up too much space. But if the expanded view is only 10% larger, why even bother providing the option to enlarge?

S & T enlarges article thumbnails to make the bigger images, because the expanded images always show worse detail (as the orbital diagram does; even the thumbnail’s text is smeared). Whoever does web graphics at S & T, you need to start with TWO images—a full sized image comfortable for people with less-than-perfect vision to easily read/decipher, then from that large image make another one, reduced in size, to use as a thumbnail that links to the big one.

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Roger Venable

October 9, 2018 at 8:59 am

This object is predicted to pass in front of a star on Friday night, October 19-20, at about 05:06 UT. Observers who can time this occultation can possibly contribute to the accuracy of its orbit and its size, in the event of an occultation. The uncertainty in the time of the event is about 11 minutes, and observers should record for 2 or 3 sigma before and after the predicted event -- I plan to record for a total of an hour. The cross-path uncertainty is greater than the diameter of the Earth, and since the object is suspected to have a diameter of only about 300 km, the chance of actually observing an event at a given site is about 0.6% at best. So, there is a low chance that a single observer will record an occultation, but a group of observers will be spread across North and South America, and together will have a reasonable chance. The importance of such an occultation makes the effort worthwhile.

A diametral occultation is expected to last about 12.5 seconds. An integration time of 2 seconds or less will be fine, but the fastest frame rate that will display the occultation star is desired. An IOTA member found a couple nights ago that he can easily image the 14.5 magnitude target star with an 8-inch Schmidt-Cassegrain telescope with an integration time of 2 seconds. The star will appear to bliink out if it is occulted, and the occulting body will not be visible.

The magnitude 14.5 star has the designation 4UCAC 517-000356. It easy to find, even by star hopping, as it is 2 degrees straight south of gamma Pegasi. Its J2000 coordinates are RA 00h 12m 18s, Dec +13d 15m, 08s. There has been an active discussion of the event on the IOTA list serve at .

The prediction has been made by Bruno Sicardy and the Lucky Star team. All results, whether "hits" or "misses", are important and should be reported to the team. See the website at and follow links to find reporting information.

-- Roger Venable

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