Every now and then, astronomers looking for alien life will spot a planet that checks a lot of boxes.
Is it in the "Goldilocks zone" — in other words, does it orbit not too far and not too close from its host star? Check.
Is there a possibility of water, in some form or other? Check.
Ahh, but that temperamental star it's orbiting is far too ornery. Exoplanets, as planets outside of our solar system are called, don't fare well in the face of blustery red suns. Harsh, ultraviolet flares obliterate anything that might aspire to live on them.
And so the search for potentially habitable worlds moves on to the next grain of sand in the star-studded beach we call the Milky Way.
But what if life on some of those planets evolved to withstand those UV blasts?
That's the question scientists at Cornell University pose in a study published in Monthly Notices of the Royal Astronomical Society.
And they think they have an answer.
It's called biofluorescence, a defense mechanism we see triggered by the sun here on our own planet.
"On Earth, there are some undersea coral that use biofluorescence to render the sun's harmful ultraviolet radiation into harmless visible wavelengths, creating a beautiful radiance," study co-author Lisa Kaltenegger, an astronomer at the Carl Sagan Institute at Cornell University, explains in a statement. "Maybe such life-forms can exist on other worlds too, leaving us a telltale sign to spot them."
If that theory proves true, it could vastly expand the search for life in our galaxy. We may even have to go back and double-check some of the glow-in-the-dark marbles found orbiting unstable stars.
Consider for example, Proxima b. Discovered in 2016, and only 4.24 light-years from Earth, this Earth-like planet might host life — if not for that UV-spitting sun. But could life here shield itself, like coral, with biofluorescence?
"These biotic kinds of exoplanets are very good targets in our search for exoplanets, and these luminescent wonders are among our best bets for finding life on exoplanets," Jack O'Malley-James, the study's lead author, notes in the statement.
A planetary call and response
Think of it as a visual game of Marco Polo. A sun belts out a flare. Marco.
It strikes the planet and triggers a warm, soft glow from whoever may be living there. Polo.
And peering through telescopes, scientists exclaim, "Got you!" Followed, of course, by a chorus of oohs and ahhs. (Because a painted planet, literally aglow with life, will make you do that, even if you're a scientist.)
The biofluorescence would only flicker briefly, but it could be enough for Earthlings to spot. Especially when they're already peering at M-type stars. Also known as red dwarfs, those are the most common stars in our universe, and they happen to host a lot of planets in their Goldilocks zone.
Unfortunately, they also happen to occasionally spew annihilation in the form of solar flares. The study suggests those flares could act more like a paintbrush tagging hidden biospheres for astronomers.
"This is a completely novel way to search for life in the universe," O'Malley-James said. "Just imagine an alien world glowing softly in a powerful telescope."
Of course, they will need to wait a little longer before they can put that theory into practice. At least until the next generation of space- or Earth-based telescopes are online. But the new, more powerful eyes on the sky aren't far off. The James Webb Space Telescope is slated for a March 2021 launch.
Three times the size of Hubble, the James Webb Space Telescope will be able to gaze deeper into space. (Photo: Northrop Grumman/NASA)
With the ability to pry deep into space — and special equipment for sniffing out planets with atmosphere — the James Webb Telescope could reveal a bold new universe.
And, perhaps even, one that shimmers with life.
Watch Lisa Kaltenegger, director of Cornell University's Carl Sagan Institute, explain why studying bioluminescence on Earth can guide us in the search for life on other planets.