An ‘interstellar object’ comes calling

An artist’s impression of the first interstellar visitor ‘Oumuamua.

‘Oumuamua, the first interstellar object discovered in the solar system in October 2017, is moving away from the sun faster than expected. This anomalous behaviour has been detected by a worldwide astronomical collaboration that studied its motion from ground-based and space-based observatories. The gain in speed is small and ‘Oumuamua is still slowing down because of the sun’s gravity but not as fast as predicted by physics. Accordingly, scientists have postulated that ‘Oumuamua is most likely an interstellar comet and not an asteroid as earlier classified. The discovery appears in the latest issue of Nature.

‘Oumuamua was first discovered using the Pan-STARRS telescope at the Haleakala Observatory, Hawaii. The word means “scout” in Hawaiian. The original observations indicated a 230-metre-long, elongated, tiny object whose colour was similar to that of a comet and whose extremely eccentric orbit too led the International Astronomical (IAU) to classify it as one. However, when astronomers could find neither gas and dust surrounding the object nor the characteristic tail, they changed its status to an interstellar asteroid. In November 2017, the discovery was reclassified as the first-ever “interstellar object”.

The team of scientists, led by Marco Micheli of the European Space Agency (ESA), explored several scenarios to explain the new observations. The most likely explanation is that ‘Oumuamua is outgassing material from its surface due to solar heating. The thrust from this ejected material is believed to provide the small but steady push that is sending it hurtling out of the solar system faster than expected—as of June 1, it was travelling at roughly 1,14,000 km an hour. Such outgassing is typical of comets and contradicts the previous classifications. “We think this is a tiny, weird comet,” said Micheli. “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.” However, the research team could not detect any visual evidence of outgassing. “We did not see any dust, coma, or tail, which is unusual,” explained Karen Meech, a co-author from the University of Hawaii. “We think that ‘Oumuamua may vent unusually large, coarse dust grains,” she added. The team has speculated that the small dust grains that adorn the surface of most comets may have, in ‘Oumuamua’s case, eroded during its journey through interstellar space, and that only the larger dust grains remained. A cloud of these large particles would not be bright enough to be detected. But it would explain the unexpected change in its speed.