objects from Oort cloud which looks like black dots on a gray background
Images of P/2013 P2, left, and P/2014 S3

Astronomers announced the discovery of two unusual objects in comet-like orbits that originate in the Oort cloud but with almost no activity—giving scientists a first look at their surfaces. These results, presented at the annual meeting of the Division of Planetary Sciences of the American Astronomical Society in Arizona in November are particularly intriguing because the surfaces are different from what astronomers expected. They are clues about the movement of material in the early solar system as the planets were assembled.

On August 4, 2013, an apparently asteroidal object, C/2013 P2 Pan-STARRS, was discovered by the Pan STARRS1 survey telescope on Haleakala. What made this object unique is its orbit—that of a comet coming from the Oort cloud, with an orbital period greater than 51 million years, yet no cometary activity was seen.

The Oort cloud is a spherical halo of comet nuclei in the outer solar system that extends to about 100,000 times the Earth-sun distance, which is known as 1 astronomical unit, or 1 AU.

“Objects on long-period orbits like this usually exhibit cometary tails, for example comet ISON and comet Hale Bopp, so we immediately knew this object was unusual,” explained team leader Karen Meech of the University of Hawaiʻi at Mānoa’s Institute for Astronomy. “I wondered if this could be the first evidence of movement of solar system building blocks from the inner solar system to the Oort cloud.”

While the team analyzed their observations of comet C/2013 P2 Pan-STARRS, a second object was discovered. C/2014 S3 Pan-STARRS was discovered through the NASA-sponsored Near Earth Object Survey on the PS1 telescope on September 22, 2014. Like C/2013 P2 Pan-STARRS, it was on the same type of cometary orbit and also showed minimal activity.

“I’ll be thrilled if this object turns out to have a surface composition similar to asteroids in the inner part of the asteroid belt. If this is the case, it will be remarkable for a body found so far out in the Solar System, especially since it exhibited a tail that may be due to volatile outgassing,” commented Meech. “There are several models that try to explain how the planets grew in the early solar system, and some of these predict that material formed close to the sun could have been thrown outward into the outer Solar System and Oort cloud, where it remains today. Maybe we are finally seeing that evidence.”

The report was presented by Karen Meech, Jan Kleyna, Jacqueline Keane and Richard Wainscoat of the University of Hawaiʻi at Mānoa’s Institute for Astronomy; Bin Yang and Olivier Hainaut from the European Southern Observatory; Henry Hsieh from Academia Sinica; Ryan Park and James Bauer from the Jet Propulsion Laboratory; Peter Veres from Comenius University and Bhuwan Bhatt and Devendra Sahu from the Indian Institute of Astrophysics.

Read the University of Hawaiʻi at Mānoa’s Institute for Astronomy news release for the full story.

—By Louise Good