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Planets Sometimes Blow Their Tops

Atmospheres, like love, often don't last forever. That's the lesson we astronomers are learning (well, at least, the atmosphere part), as we push outward with our telescopes into a galaxy rich with planets.

It's not an insignificant point, since the fate of atmospheres holds the key to science's most enduring question: Are we alone in the universe?

Last Monday, I spent the day at Penn State working with James Kasting on how planets can lose their atmospheres into space. Kasting is a great scientist who has spent much of his career exploring what allows a planet to become habitable. In astronomy, a planet is habitable if liquid water can exist on its surface. To answer his habitability questions, Kasting uses detailed models of planetary atmospheres that include the absorption of sunlight, the effect of ozone, the action of greenhouse gases and much more.

But, as Kasting will tell you, it's not just what happens inside the atmosphere that matters for keeping a planet safe for life.

What happens high up at the border with interplanetary space can also make the difference between a living or dead world. That's because planets can bleed their atmospheres into space. This can happen slowly, in a trickle, or quickly when a planet blows its top in a powerful wind. In either case, this atmospheric evaporation can shape a world's long-term fate.

For rocky "terrestrial" planets like Earth, Venus and Mars, the atmosphere constitutes a relatively thin blanket of gas. On Earth, the lower layers of the atmosphere extend up to about 50 km (around 31 miles). This includes the layers where most of our weather occurs (the Troposphere — below 12 km, or 7.5 miles). But the atmosphere's upper layers reach all the way out to 500 km (310 miles) or more above the surface. There, at the ragged edge of Earth's gaseous veil, sunlight heats atoms like hydrogen to high temperatures (and high speeds) allowing them to escape into space. While this slow trickle of atmospheric constituents does not affect Earth very much, over time it has completely altered our neighbor worlds of Venus and Mars.

The Venus of today is a hell-planet with surface temperatures above 800 F (high enough to melt lead). It didn't start out this way, though. Venus lost all its water through atmospheric escape early on (the water molecules were zapped apart by sunlight and the hydrogen atoms escaped). That left the atmosphere with an overabundance of CO2, a potent greenhouse gas. It was the depletion of water via particles skipping out the atmosphere's top door that doomed Venus.

Losing atmosphere to space is also what killed Mars. The red desert world we see today was, most likely, once a blue planet with a thick atmosphere blanketing large seas and wide rivers. But Mars is less massive than Earth. That means its gravity is weaker and atoms at the top of its atmosphere have an easier time escaping. Like a Macy's parade balloon with a slow leak, Mars' atmosphere has slowly deflated over millennia, leaving it the cold, desiccated world we find today.

But atmospheric escape isn't just an issue for planets in our own solar system. Since 1995, we've been discovering planets around other suns at an astonishing rate. Our observations of these distant worlds have become pretty sophisticated. So, now we've begun unpacking their properties in detail (which is pretty insane when you think about it). Among the more astonishing insights we've gained is that some worlds are stripping themselves down through massive evaporations in the form of a "planetary wind."

While Earth dwells at a comfortable distance from the sun, there are many exoplanets on "hot" orbits. These worlds are so close to their stars that their "years" are measured in our days.

Such tiny orbits mean radiation from the star sears the planet's atmosphere, heating it to temperatures as high as 10,000 degrees. With so much intense starlight flooding through the sky, the atmosphere is jolted into movement, flowing outward into space at speeds over 150,000 miles per hour. The planetary wind streams out into the space between the planet and the star curling into beautiful and complex orbits (see the video).

The Hubble Space Telescope has already given us strong evidence for the existence of powerful planetary winds from at least two Jupiter-sized planets on hot orbits. "Hot Neptunes," which are less massive than Jupiters, also exist, and the winds from these worlds might even be change agents. A strong enough wind from a hot Neptune might entirely strip the atmosphere away, leaving only a charred rocky core in its wake.

So, a planet's winds may slowly bleed a world of its chance as a safe haven for life — or the winds may boil the world away. In either case, its just another reason to look up at the sky — day or night — and remember this is really a pretty amazing universe.

Adam Frank is a co-founder of the 13.7 blog, an astrophysics professor at the University of Rochester, a book author and a self-described "evangelist of science." You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Adam Frank was a contributor to the NPR blog 13.7: Cosmos & Culture. A professor at the University of Rochester, Frank is a theoretical/computational astrophysicist and currently heads a research group developing supercomputer code to study the formation and death of stars. Frank's research has also explored the evolution of newly born planets and the structure of clouds in the interstellar medium. Recently, he has begun work in the fields of astrobiology and network theory/data science. Frank also holds a joint appointment at the Laboratory for Laser Energetics, a Department of Energy fusion lab.

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