Asteroid, comet, alien spaceship? Scientists have new theory on what mysterious interstellar object Oumuamua is
Researchers at the University of Hawaii first discovered ‘Oumuamua in 2017, more than a month after it passed its closest point to the Sun. They named the object after the Hawaiian word for 'scout'
‘Oumuamua, the first known interstellar object to pass through the Earth’s solar system, has been called many things: an asteroid, a comet, a cigar-shaped spaceship. Now it has a new description: astronomers at Yale University and the University of Chicago say it could be a hydrogen iceberg. This is a new type of object and it is likely that a large population of similar objects exist, says the research team.
‘Oumuamua is about 900 feet long and traveled through space for millions of years before entering the solar system. It has now passed beyond Saturn’s orbit and will travel another 10,000 years before exiting the system.
“We developed a theory that explains all of ‘Oumuamua’s weird properties. We show that it was likely composed of hydrogen ice. This is a new type of object, but it looks like there may be many more of them showing up, going forward,” says co-author Gregory Laughlin, a professor of astronomy at Yale’s Faculty of Arts and Sciences, in the analysis that appears on the preprint website arXiv. The study has been accepted for publication by the Astrophysical Journal Letters.
“It’s a frozen iceberg of molecular hydrogen. This explains every mysterious property about it. And if it’s true, it’s likely that the galaxy is full of similar objects,” says the first author of the study, Darryl Seligman, a former Yale graduate student now at the University of Chicago, who began the research at Yale. Researchers at the University of Hawaii first discovered ‘Oumuamua in 2017, more than a month after it passed its closest point to the Sun. They named the object after the Hawaiian word for “scout.” It was spotted by the Panoramic Survey Telescope and Rapid Response System 1 (PanSTARRS1) telescope, located at the University of Hawaii's Haleakala Observatory and made headlines as the first object to visit from outside the solar system.
Telescopes did not pick it up until it had already swung past the sun and was on its way out, but its trajectory indicated it came from interstellar space. It was ‘Oumuamua’s odd and unique characteristics that had led to widespread interest and speculation the object could be an alien spacecraft, sent from a distant civilization to examine our star system. Astronomers formally named the object 1I/2017 U1 and appended the common name ‘Oumuamua. A previous study had stated that ‘Oumuamua likely has an elongated, cigar-like shape and an odd spin pattern, much like a soda bottle laying on the ground, spinning on its side.
As ‘Oumuamua hurtled through the inner part of the solar system, astronomers noticed it had several unusual properties. It varied rapidly in brightness, suggesting it was either saucer-shaped or cigar-shaped. It also accelerated in a fashion similar to a comet — yet it showed no evidence of emitting gas or the fine billows of dust normally associated with comets. The typical comets scientists have seen in our solar system have cometary tails, which can be seen when small dust particles in the outflow reflect sunlight, but they could not see any such sparkle of dust from ‘Oumuamua. Last year, however, the research team showed that it could be a comet whose outflow was simply invisible to telescopes. Starting from that idea, the researchers worked their way backward to see what the substance could be in the outflow. They knew where ‘Oumuamua was, how fast it was moving and how much energy it should be getting from the sun at any given time, so they checked the list of what materials would give the acceleration they saw when burned up.
According to scientists, ‘Oumuamua’s behavior can be explained if it is composed of hydrogen ice. “The only kind of ice that really explains the acceleration is molecular hydrogen,” says Seligman. Molecular hydrogen ice is a strange substance, only formed when the temperature is just a little above absolute zero. It does not reflect light or produce any light as it burns up, so telescopes would not be able to see it. “While hydrogen is the most common element in the universe, it is rarely found in solid form, which requires extremely cold temperatures. Frozen hydrogen does, however, offer a compelling mechanism for acceleration,” says the study.
Laughlin elaborates, that as ‘Oumuamua passed close to the Sun and received its warmth, “melting hydrogen would have rapidly boiled off the icy surface, providing the observed acceleration and also winnowing ‘Oumuamua down to its weird, elongated shape — much as a bar of soap becomes a thin sliver after many uses in the shower.” Seligman argues that since they saw one at all implies that the galaxy must be filled with these dark hydrogen icebergs.
The study theorizes that iceberg-like objects made of hydrogen can potentially form in the dense cores of molecular clouds that pervade the Milky Way galaxy and give rise to new stars and planetary systems. “Their presence would be an accurate probe of the conditions in the dark recesses of star-forming clouds and provide a critical new clue for understanding the earliest phases of the still-mysterious processes that generate the birth of stars and their accompanying planets,” says Laughlin.