Most Interstellar Objects Might Evaporate Between Stars

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In 2017, astronomers around the world turned their attention to a small point of light zipping through the solar system. The object, known as ‘Oumuamua, was the very first interstellar object ever discovered. We have since spotted a second, suggesting that visitors from afar might be common. However, a new analysis claims that almost all the potential visitors drifting between stars evaporate long before they could get here. 

We still don’t have a good handle on what ‘Oumuamua is. Assuming it’s not an alien spaceship, it’s probably composed primarily of ice — whether that ice is water, nitrogen, methane, or something else is still up in the air, though. The new research, led by Vo Hong Minh Phan from Aachen University in Germany, posits that ‘Oumuamua and similar icy objects would be susceptible to cosmic ray erosion that shrinks them down to nothing. That could mean that only the largest interstellar objects survive long enough to cross into an alien solar system. 

Cosmic rays consist of atomic nuclei traveling at nearly the speed of light, and they can cause significant damage to biological materials. They’re produced by energetic events like supernovae, but luckily, most of the high-energy particles are blocked by Earth’s magnetic field. For an object like ‘Oumuamua traveling between stars, cosmic rays could slowly chip away at the structure. It would take ages to erode a mountain of ice, but the journey between stars is no quick trip down the block. 

The Milky Way, as seen from here on Earth

Even without knowing for certain what ‘Oumuamua was, the team was still able to make some general predictions. If the object was largely composed of nitrogen ice — for example, if it was a fragment of a Pluto-like dwarf planet — it probably started much larger. Current evidence suggests ‘Oumuamua originated in the Perseus arm of the Galaxy some 500 million years ago. That could mean it was initially 10-50 kilometers in diameter, and what we saw flying past was just a tiny remainder. We don’t know the exact size now, but it’s on the order of a few hundred meters at most. 

A faster-moving interstellar object wouldn’t necessarily make it to us intact, either. Even in deep space between stars, there is diffuse gas known as the interstellar medium (ISM). A chunk of ice moving faster will experience more erosion from colliding with the ISM, shaving it down little by little over the eons. 

This research is still in the early stages, and there are plenty of unanswered questions about this new class of alien objects. It might be harder to learn about them, too, if most end up evaporating in between stars. Several projects hoping to identify and characterize interstellar interlopers are underway, but we’re still years away from any significant searches.

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