A few weeks ago, something surprising happened when astronomers noticed an odd object moving quickly through the Solar System. Being on a large looping trajectory, it was first thought to be a previously unknown comet, but then calculations showed that it couldn't have originated from within the Solar System, it must have come from somewhere else. Follow-up observations also showed that it was more like an asteroid, rather than a comet. Now, astronomers have published their most detailed findings yet, and this object, named 'Oumuamua (Hawaiian, meaning scout or messenger coming from the past), is "like nothing seen before."
The object was first seen on Oct. 19, 2017 by the Pan-STARRS 1 telescope in Hawaii, operated by the European Space Agency (ESO) and was then observed by multiple telescopes around the world. It had already passed closest to the Sun in its orbit and was now heading out of the Solar System again, so astronomers had to act fast to see it.
“We had to act quickly,” said team member Olivier Hainaut from ESO in Garching, Germany. "'Oumuamua had already passed its closest point to the Sun and was heading back into interstellar space."
Combined images of ‘Oumuamua
Light curve of ‘Oumuamua
Astronomers were able to measure both the object's brightness and colour with ESO's Very Large Telescope in Chile. By combining images from the FORS instrument on the VLT using four different filters, with those of other large telescopes, astronomers found that 'Oumuamua varied dramatically in brightness by a factor of ten as it spun on its axis every 7.3 hours.
As Karen Meech noted, “This unusually large variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape. We also found that it has a dark red colour, similar to objects in the outer Solar System, and confirmed that it is completely inert, without the faintest hint of dust around it.”
In other words, this odd interstellar asteroid is shaped like a giant cigar, something not typically seen in our Solar System, and is estimated to be about 400 metres (1,312 feet) long. 'Oumuamua appears to be dense, rocky or with high metal content, has little or no water or ice and a surface darkened and reddened due to the effects of irradiation from cosmic rays over millions of years.
“What a fascinating discovery this is!” said Paul Chodas, manager of the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory, Pasadena, California. “It’s a strange visitor from a faraway star system, shaped like nothing we’ve ever seen in our own solar system neighborhood.”
Trajectory of ‘Oumuamua
“We are continuing to observe this unique object,” said Hainaut, “and we hope to more accurately pin down where it came from and where it is going next on its tour of the galaxy. And now that we have found the first interstellar rock, we are getting ready for the next ones!”
Analysis of its orbit indicates that it most likely originated from the direction of the star Vega, traveling at around 95,000 kilometres/hour (59,030 miles/hour).
This is the first time that such an interstellar asteroid has been seen entering our Solar System, but astronomers estimate that there may be other ones about once per year or so.
"We have long suspected that these objects should exist, because during the process of planet formation a lot of material should be ejected from planetary systems. What's most surprising is that we've never seen interstellar objects pass through before," said Meech.
"We have been waiting for this day for decades," said CNEOS Manager Paul Chodas. "It's long been theorized that such objects exist - asteroids or comets moving around between the stars and occasionally passing through our solar system - but this is the first such detection. So far, everything indicates this is likely an interstellar object, but more data would help to confirm it."
The discovery is reminiscent of the science fiction classic Rendezvous with Rama by Arthur C. Clarke where a huge cylindrical alien vessel enters the Solar System, initially mistaken for an asteroid. In this case, the object is probably natural based on what is known about it so far, but it is weird.
The new paper is available here (subscription/purchase).