Black Hole Outbursts Could Turn 'mini-Neptunes' Into Rocky Planets

2_28_Rocky Super-Earth
An artist’s impression shows The atmosphere of a Neptune-like planet is swept backward by powerful radiation from an outburst in the center of the Milky Way in this artist's impression. The planet’s host star is shown on the left. M Weiss/CfA

Black holes may shape a planet's destiny. According to new research, high-energy radiation outbursts from the Milky Way's supermassive black hole can remove atmospheres from nearby Neptune-like planets. When that happens, say the researchers, these planets may be left with just their rocky core intact.

"It's pretty wild to think of black holes shaping the evolutionary destiny of a planet, but that very well may be the case in the center of our galaxy," Howard Chen, a physicist at Northwestern University in Illinois, who led the study, said in a statement.

The research was recently posted to the arXiv academic preprint server.

Scientists know that when black holes like Sagittarius A* feast on nearby matter, they can release flares of X-ray and ultraviolet radiation. Chen and colleagues wondered how these powerful events affect nearby planets.

The team looked at planets located within 70 light years of Sagittarius A* that had masses measuring between that of Earth and that of Neptune. The outbursts of radiation, the researchers found, could wipe out large portions of the atmospheres surrounding these planets.

That kind of destruction could have huge implications. Such flares "could dramatically change a planet's life," co-author John Forbes from the Harvard-Smithsonian Center for Astrophysics (CfA) explained in the statement.

Read more: How Does a Black Hole Burp? Belching Supermassive Black Hole Has Big Implications for Physics

Larger than Earth, the planets investigated in the study are called "super-Earths." Astronomers have found an abundance of these planets in our galaxy, and many scientists have focused on understanding how they take shape. "Our work shows that in the right environment they might form in exotic ways," said co-author Avi Loeb, also of CfA.

The team thinks black hole outbursts could be one of the most frequent ways these super-Earths are created near the Milky Way's center.

Alien-hunting in a crowded space

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An artist illustrates the K2-18 planetary system, which contains super Earths. Alex Boersma

Because some super-Earths are a target for alien-hunters, the researchers also speculated about whether those sitting near Sagittarius A* could potentially support life. Some planets, they wrote, would be in the so-called habitable zones of nearby stars—that is, just the right distance away to support conditions for life.

But proximity to its star isn't the only important factor when it comes to finding life. How populated the region of space is where the planet is found also matters.

The super-Earths examined in this study are in a crowded area. The average separation between rocky planets there is thought to be as small as 45 billion miles. For comparison, our solar system's nearest neighboring star is some 25,000 billion miles away.

This high density could stifle a planet's ability to support life. The super-Earths could be in the path of exploding stars and gamma ray bursts. Or they could be catapulted from their host stars by gravitational disruptions from other stars passing by, forcing them out of a habitable zone.

Read more: Black Holes: Clearest Infrared Milky Way Image Reveals Galaxy's Magnetic Fields

Even if a planet avoided the problems of a crowded patch of space with some of its atmosphere intact, it could be still zapped by further black hole emissions, the researchers wrote.

That being said, any life that did appear in this region could well hitch a ride on nearby matter, the authors suggest. That possibility rests on a theory known as panspermia, which holds that life originate in one place and be brought elsewhere on, say, an asteroid.

Although Loeb acknowledged that life in this region is unlikely, panspermia, he said, "would be much more common in such a dense environment." And this chance of interstellar contact, said Loeb, "might give life a fighting chance to arise and survive."

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