Scott Sheppard (Carnegie Institution for Science), Chad Trujillo (Gemini Observatory), and David Tholen (University of Hawai’i) discovered a new object in the distant reaches of the Solar System last month and designated it Object V774104. V774104 lies 103 au away in the direction of west-central Pisces — that’s 9.6 billion miles or 15.4 billion km. This would put it almost three times as far away as Pluto, and even well beyond the Kuiper Belt. Sheppard announced the discovery at last week’s meeting of the Division for Planetary Sciences of the American Astronomical Society.
Prowling the outer Kuiper Belt for large, distant members of our solar system has turned up a zoo of remarkable finds in recent years. There’s Eris, for example which triggered a divisive debate about Pluto’s planetary status; Sedna, whose orbit carries it out to more than 900 astronomical units (1 a.u. is the mean Earth-Sun separation); and 2007 OR10, both very distant (87 a.u.) and one of the reddest objects in the solar system.
During an observing campaign for new trans-Neptunian objects utilising the 8.2-meter Subaru Telescope on Mauna Key and the Dark Energy Camera mounted on NOAO’s 4-meter Víctor M. Blanco Telescope in Chile, the researchers noticed last month a previously unseen bright speck of light moving slowly relative to the background stars.
V774104 announced on November 10
Sheppard announced V774104 on November 10 at a meeting held in National Harbor, Maryland. Yeah, V774104’s a weird name. There are far too many objects in space for all of them to be given real names, so the vast majority just get these strange combinations of letters and numbers to identify them with. I compiled the following information on a few of the major objects with real names:
The planets and the Major Asteroid Belt objects all have relatively circular orbits, especially those nearest the sun. The orbits of Saturn, Uranus, and Neptune are a little more elliptical than the inner five; and those of Pluto and the Kuiper Belt, even more so.
The planets all orbit near a plane radiating out from the Sun’s equator, but those farthest out tend to be a little more inclined. Pluto and Eris have inclinations of 17 degrees and 44 degrees respectively, and the large asteroid Pallas is inclined at 34 degrees. At Oort Cloud distances, many of the inclinations approach 90 degree, and the plane becomes a hollow sphere.
Planets in the Solar System revolve in their orbits in a prograde manner, i.e., in the same direction as the Sun’s rotation on its axis. With the exceptions of Venus and Uranus, they also rotate progradely on their axes; but these two have retrograde rotations.
Most satellites of planets also revolve around them in the prograde sense. (In the case of the satellites of Uranus, this means they revolve in the same sense as Uranus’s rotation, which is retrograde relative to the Sun.) The exceptions are generally small and distant from their planets, except for Neptune’s satellite Triton, which is large and close. It is believed that these retrograde satellites, including Triton, were formed in other locations and later captured by their planets.
The Oort Cloud is a theoretical hollow sphere containing trillions of icy objects too far away for us to see them yet. The reason we know about the Oort Cloud is by studying the comets that occasionally fall toward the Sun from it. The distance and location from which a body has fallen toward the sun can be estimated by its velocity.
About as far away as the Pioneer probes have gone in 43 years
At 103 AU, or 9.6 billion miles, V774104 is about as far away as the twin Pioneer probes, which have been traveling since 1972 and 1973. It’s beyond the main Kuiper Belt, but not far enough away to be considered part of the Oort Cloud. It is believed to be the most distant object ever seen in orbit around the sun yet, though we’ll undoubtedly find many more as our instruments and techniques keep on improving. (One AU is the average distance between earth and the Sun, or approximately 93 million miles.)
Anyway, Shepherd’s team found the new worldlet while exploring a section of space way out beyond Pluto to see if anything was moving there. They do this by taking pictures of exactly the same space several hours apart and comparing them. The animated gif on the right shows two such pictures. While background stars far beyond our solar system appear stationary, the newly discovered V774104 clearly jumps from one spot to another near the center of the picture. The frame is 0.7 arcminute wide.
This is not because of V774104’s own movement through the outer reaches of the solar system. At that distance from the sun, it orbits too slowly. Instead, it is earth’s movement in its own orbit that seems to make it jump. This is called parallax.
To get a simple idea of how parallax works, hold your thumb at arm’s length. First close your right eye and look at your thumb with your left eye. Next, close your left eye and look at your thumb with your right eye. Notice how your thumb seems to jump from one side to the other as you change eyes? This represents the pictures taken from two points in earth’s orbit a few hours apart. Anything in the pictures will show some parallax, but only objects in the solar system will be close enough to notice it. (Some of the closest stars show enough parallax to detect, but only if the pictures are taken six months apart from opposite sides of the planet’s orbit.)
Parallax is how asteroids, comets, and dwarf planets are nearly always discovered. It was also used originally to estimate the distance between the sun and a few of the nearest stars, before better techniques were invented.
From the amount of parallax, Sheppard’s team calculated that V774104 is about 103 astronomical units (AU) away from the sun. Then based on its brightness, they estimated its diameter to be between 300 and 600 miles (500 and 1000 km), or less than half that of Pluto. Although it’s too soon to say for sure, it appears to have an orbit maybe two or three times larger than Pluto’s. (The orbit can be determined accurately only after at least a year of observation.)
While it is clearly one of the most distant planetary bodies ever observed in the solar system, it will require more observation be sure it holds that record during its entire orbit.
The three most distant dwarf planets known all have eccentric orbits, and none of them could have formed in their current locations, Sheppard says. They may have been perturbed into these orbits, either by an encounter with another star in the solar system’s early years, or by a still undiscovered Mars- or Earth-sized planet lurking in the outer solar system. “We can’t explain these objects’ orbits from what we know about the solar system,” he said.
The discovery reflects a number of extreme solar system surveys that are using telescopes with both big mirrors and large fields of view—necessary to find faint solar system objects that could be almost anywhere in the sky. Unlike many searches for distant objects, which peer into the solar system’s plane, Sheppard is training Subaru on swaths of the sky an average of 15° away from the ecliptic, the better to find other weird objects.
“We want to find a bunch of these objects like VP113 we found last year,” Sheppard said. “There’s several different theories about how these distant objects could have got out there on these eccentric orbits. And all these different theories predict different orbital distribution and orbital population. So if we can find 10 or so of these objects, then we can start determining which theories of the formation of these objects are correct.”
The diagram above, from Wikipedia, shows the relative locations of several parts of the Solar System. The new object is too new to appear here, but it would be off the edges anyway. The outer edges of the Oort Cloud (if shown) would extend hundreds of feet out beyond the edges of the picture in all directions.