1I/2017 U1 ‘Oumuamua — the first known object of extrasolar origin discovered within the Solar System — has been the subject of intense scrutiny since its discovery in 2017. Now, by combining data from several space- and ground-based telescopes, astronomers have found that ‘Oumuamua is moving faster than predicted. The results, published in the journal Nature, suggest that the interstellar object is most likely a comet and not an asteroid.
This artist’s impression shows ‘Oumuamua, the first interstellar object discovered in our Solar System. Recent observations show that the object is moving faster than predicted while leaving the Solar System. Astronomers assume that venting material from its surface due to solar heating is responsible for this behavior. 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. Image credit: NASA / ESA / Hubble / ESO / M. Kornmesser.
In October 2017, the Pan-STARRS 1 telescope in Hawai’i picked up a faint point of light moving across the sky. It initially looked like a typical small asteroid, but additional observations over the next couple of days allowed its orbit to be computed accurately.
Unlike any asteroid or comet observed before, the mystery visitor sped past the Sun, approaching from ‘above’ the plabe of the planets on a highly inclined orbit, moving fast enough to escape the Sun’s gravitational pull and eventually depart our Solar System.
Since interstellar comets are thought to be more numerous than interstellar asteroids, the object was first assumed to be a comet. 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.
Following the initial discovery, Dr. Marco Micheli of ESA’s SSA-NEO Coordination Centre and colleagues continued to make high precision measurements of ‘Oumuamua and its position using the NASA/ESA Hubble Space Telescope, ESO’s Very Large Telescope, the Canada-France-Hawai’i Telescope, ESA’s Optical Ground Station telescope, and other telescopes around the world.
Contrary to their expectations, the astronomers found that the object was not following the anticipated trajectory if only the gravity of the Sun and the planets were determining its path.
This diagram shows the orbit of ‘Oumuamua as it passes through the Solar System. It shows the predicted path of the object and the new course, taking the new measured velocity of ‘Oumuamua into account. The asteroid 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 2025, ‘Oumuamua will reach the outer edge of the Kuiper Belt, and then the heliopause — the edge of the Solar System — in November 2038. Image credit: ESA.
“Unexpectedly, we found that Oumuamua was not slowing down as much as it should have due to just gravitational forces. What could be causing this curious behavior?” Dr. Micheli said.
The analysis of the new data ruled out a range of possible influences, such as radiation pressure or thermal effects from the Sun, or interaction with the 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.
“We think this is a tiny, weird comet. We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets,” Dr. Micheli said.
Usually, when comets are warmed by the Sun they eject dust and gas, which form a cloud of material — called a coma — around them, as well as the characteristic tail. The release of gas pressure at different locations and can have the effect of pushing the comet slightly off-course compared with the expected path if only gravitational forces were at play.
However, the scientists could not detect any visual evidence of outgassing.
“We did not see any dust, coma, or tail, which is unusual. We think that ‘Oumuamua may vent unusually large, coarse dust grains,” said co-author Dr. Karen Meech, from the University of Hawaii.
The researchers speculated that perhaps the small dust grains adorning the surface of most comets eroded during ‘Oumuamua’s journey through interstellar space, with only larger dust grains remaining. Though a cloud of these larger particles would not be bright enough to be detected, it would explain the unexpected change to ‘Oumuamua’s speed.
“‘Oumuamua is small and it could have been releasing a small amount of relatively large dust for it to have escaped detection,” Dr. Meech said.
“To really understand Oumuamua 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.”
‘Oumuamua, less than half a mile in length, now is farther away from our Sun than Jupiter and traveling away from the Sun at 70,800 mph (114,000 km/h, as of July 1, 2018) as it heads toward the outskirts of the Solar System. In only another four years, it will pass Neptune’s orbit on its way back into interstellar space.
“The true nature of this enigmatic interstellar nomad may remain a mystery. ‘Oumuamua’s recently-detected gain in speed makes it more difficult to be able to trace the path it took from its extrasolar home star,” said ESO astronomer Dr. Olivier Hainaut.
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Marco Micheli et al. Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua). Nature, published online June 27, 2018; doi: 10.1038/s41586-018-0254-4
