Is ʻOumuamua Really a Comet?

This animation, based on an artist’s impression, shows the first interstellar object ʻOumuamua. Hubble found ʻOumuamua moving faster than expected. Researchers assume that venting material from its surface due to solar heating is responsible for the observed speed. The animation also shows the tumbling motion of the object. Credit: ESA/Hubble, NASA, ESO, M. Kornmesser

The interstellar object ʻOumuamua was discovered back on Octobter 19, 2017, but the puzzle of its true nature has taken months to unravel, and may never be fully solved.

Meaning ‘scout from the distant past’ in Hawaiian, ‘Oumuamua was found by astronomers working with the University of Hawaiʻiʻs Pan-STARRS1 survey as it came close to Earth’s orbit. But what is it? an asteroid, a comet or a rock that just happens to be passing by? As soon as it was spotted, astronomers from around the world were on the case.

The first clue: its trajectory. Extensive follow-up observations by the Canada-France-Hawaii Telescope (CFHT), the European Space Agency’s (ESA) Optical Ground Station telescope in Tenerife, Canary Islands, and other telescopes around the world have helped pin it down.

‘Oumuamua was first spotted about a month after its closest approach to the Sun, which took it within the orbit of Mercury. Unlike any asteroid or comet observed before, this new object sped past the Sun, approaching from ‘above’ the plane of the planets on a highly inclined orbit, moving fast enough (70,800 miles per hour as of July 1, 2018) to escape the Sun’s gravitational pull and eventually depart our Solar System.

Initially, astronomers assumed ‘Oumuamua was a comet.  Current understanding of planet formation predicts more interstellar comets than interstellar asteroids.  However, astronomers did not see evidence of gas emission or a dusty environment in the observations. Without these hallmarks of cometary activity, it was classified as the first interstellar asteroid.

But the story has another surprising twist.

Following the initial discovery observations with Pan-STARRS, a team of astronomers led by Marco Micheli of ESA’s SSA-NEO Coordination Centre, and Karen Meech of the University of Hawai‘i Institute for Astronomy, continued to make high precision measurements of the object and its position using many ground-based facilities like CFHT, as well as the Hubble Space Telescope. The final images were taken with Hubble in January, before the object became too faint to observe as it sped away on its outbound orbit.

This diagram shows the orbit of the interstellar object ‘Oumuamua as it passes through the Solar System. It shows the predicted path of ‘Oumuamua and the new course, taking the new measured velocity of the object into account. ‘Oumuamua passed the distance of Jupiter’s orbit in early May 2018 and will pass Saturn’s orbit January 2019. It will reach a distance corresponding to Uranus’ orbit in August 2020 and of Neptune in late June 2024. In late 2025 ‘Oumuamua will reach the outer edge of the Kuiper Belt, and then the heliopause — the edge of the Solar System — in November 2038. Credit: ESA

Contrary to their expectations, the team found that the object was not following the anticipated trajectory if only the gravity of the Sun and the planets were determining its path. "Unexpectedly, we found that ‘Oumuamua was not slowing down as much as it should have due to just gravitational forces", says Marco, lead author of the paper reporting the team's findings,published today in the journal Nature. What could be causing this curious behavior?

Rigorous analysis ruled out a range of possible influences, such as radiation pressure or thermal effects from the Sun, or interaction with the Sun’s solar wind. Other, less likely scenarios, such as a collision with another body, or ‘Oumuamua being two separate, loosely held-together objects, were also discarded

Comets contain ices that sublimate, or turn directly from a solid to a gas when warmed by the Sun. This process drags out dust from the comet’s surface to create a fuzzy ‘atmosphere’ and sometimes a tail. The release of gas pressure at different locations and times can have the effect of pushing the comet slightly off-course compared with the expected path if only gravitational forces were at play.

This artist’s impression shows the first interstellar object discovered in the Solar System, ʻOumuamua. Observations made with the NASA/ESA Hubble Space Telescope and others show that the object is moving faster than predicted while leaving the Solar System. Researchers assume that venting material from its surface due to solar heating is responsible for this behaviour. This outgassing can be seen in this artist’s impression as a subtle cloud being ejected from the side of the object facing the Sun. As outgassing is a behaviour typical for comets, the team thinks that ʻOumuamua’s previous classification as an interstellar asteroid has to be corrected. Credit: ESA/Hubble, NASA, ESO, M. Kornmesser

“Thanks to the high quality of the observations we were able to characterize the direction and magnitude of the non-gravitational perturbation, which behaves the same way as comet outgassing” says Davide Farnocchia of NASA’s Jet Propulsion Laboratory.    

The team has not detected any dusty material or chemical signatures that would typically characterize a comet, even in the deepest images from ESO’s Very Large Telescope, Hubble and the Gemini South telescope.“ ʻOumuamua is small-- no more than a half a mile long-- and it could have been releasing a small amount of relatively large dust for it to have escaped detection,” said Meech.  “To really understand ʻOumuamua we would need to send a space probe to it. This is actually possible - but it would be very expensive and take a long time to get there, so it isn’t practical this time.  We just have to be ready for the next one.”

“It was extremely surprising that ʻOumuamua first appeared as an asteroid, given that we expect interstellar comets should be far more abundant, so we have at least solved that particular puzzle,” says Olivier Hainaut of the European Southern Observatory.  “It is still a tiny and weird object that is not behaving like a typical comet, but our results certainly lean towards it being a comet and not an asteroid after all.”

Because of its small size and faintness, current observations of ‘Oumuamua do not provide all the information astronomers need to determine important aspects of the comet’s surface.  “When ‘Oumuamua was discovered, the astronomy community gathered as much data as possible, but ultimately, the object was just not visible long enough to answer all our questions, “ says Ken Chambers from PanSTARRS.  “With PanSTARRS monitoring the skies, we hope to discover more ‘Oumuamua-like objects in the future and begin to answer the really interesting questions about this class of objects.”


Additional information

University of Hawaii, Institute for Astronomy News Release
Hubble Space Telescope News Release
European Space Agency News Release
Dr. Karen Meech ʻOumuamua TED talk
European Southern Observatory News Release

Paper- "Non-gravitational acceleration in the trajectory of 1I/2017 U1 (ʻOumuamua)

Contact Information:

Media contacts
Mary Beth Laychak, Outreach manager
Canada-France-Hawaii Telescope

Science contacts
Karen Meech
UH Institute for Astronomy

Marco Micheli
Space Situtational Awareness Near-Earth Object Coordination Center
European Space Agency, Frascati, Italy