Two travelers from far beyond Neptune return home

TUCSON, Ariz. — Two visitors from the edge of the solar system appear to be returning to their birthplace. One is made of rock, the other slathered in organic compounds; neither looks like other bodies from the Oort cloud, the icy debris field that envelops the solar system. The objects may be relics from the solar system’s formative years, thrown to the Oort cloud while the planets were still forming over 4 billion years ago.

One body, designated C/2013 P2 Pan-STARRS, is making a rare appearance as it loops around the sun once every 51 million years. Karen Meech, a planetary scientist at the University of Hawaii in Honolulu, and colleagues discovered the object in August 2013, when it was about three times as far from the sun as Earth is.

The object’s highly stretched orbit indicates that C/2013 P2 most likely came from the Oort cloud.  But the lack of a tail immediately flagged C/2013 P2 as an oddball. Comets from the Oort cloud, like the recent visitors ISON (SN: 11/16/13, p. 14) and Siding Spring (SN Online: 10/20/14), typically light up as they approach the sun. Heat from the sun generates long, bright tails of ice and dust. But C/2013 P2 showed only a barely visible stream of particles trailing behind.

“This was not acting like anything we have seen,” said Meech, who presented the find November 11 at the American Astronomical Society’s Division for Planetary Sciences meeting.

While investigating C/2013 P2, Meech and colleagues stumbled across another body passing through the asteroid belt that is also on a cometlike orbit but bearing little ice or dust. The second object, designated C/2014 S3 Pan-STARRS, loops around the sun once every 314,000 years along an orbit that takes it roughly 27 billion kilometers from the sun, or six times farther than Neptune.

Meech and colleagues took a closer look at both objects with the Canada-France-Hawaii Telescope on Mauna Kea in Hawaii. P2 is red, indicating it is covered in organic material, whereas S3 appears to be made of rock. Since P2 looks like objects in the Kuiper belt, the ring of ice boulders just beyond Neptune, and rocky S3 looks like asteroids, which mainly inhabit the inner solar system, both must have formed much closer to the sun than the Oort cloud.

Scott Sheppard, a planetary scientist at the Carnegie Institution for Science in Washington, D.C., says the composition of C/2014 S3, in particular, is surprising. Asteroids form only inside the orbit of Jupiter. For a rock to arrive from the backwaters of the solar system, it must have been launched out there long ago. After spending billions of years in the Oort cloud, a gravitational nudge might have sent it falling back toward the sun. A close encounter with Jupiter could then have redirected C/2014 S3 into its current orbit.

The scenario is speculative. But if it’s right, C/2014 S3 could be a remnant from the solar system’s first few million years, when Jupiter is thought to have wandered toward the sun, into the current orbit of Mars, before heading back out. That theory, known as the “Grand Tack model,” nicely explains the surprisingly small size of Mars and the locations of objects within the asteroid belt (SN: 5/5/12, p. 24). But there’s little direct evidence to support it.

Meech says that if the Grand Tack model is correct, then researchers can pinpoint the origin of Earth’s water (SN Online: 11/1/14). As Jupiter plowed through the inner solar system, it should also have dragged along icy asteroids from the outer solar system that eventually plowed into Earth. Alternatively, Earth may have formed entirely from local ingredients.

“This has major implications for forming habitable worlds,” said Meech. If Earth needed Jupiter to lob water at it, that may mean that life on extrasolar planets depends on giant planets setting the stage.

To test these ideas, researchers need to figure out if C/2013 P2 and C/2014 S3 are unique, or if there are many more objects like them. If planetary scientists find that a substantial number of Oort cloud objects look similar, that would imply that Jupiter did migrate and toss around lots of planetary detritus, which would provide researchers with an indirect way of seeing how the solar system behaved over 4 billion years ago.