This Planet is Too Hot to Hold Alien Life — But Just Gave Astronomers Hope for It Elsewhere
Here's what that means for the search for life.
A recent discovery on a rocky little world [this far] away may offer some hope in the search for habitable exoplanets — even though the planet in question is not, itself, habitable.
YZ Ceti b orbits much too close to its star to hold liquid water on its surface, but a team of astronomers led by Pineda and Vassar College astronomer Jackie Villadsen say it may have a magnetic field that interacts with its star, causing the system to blast radio waves out into space. And a rocky exoplanet with a magnetic field could point to one small piece of the answer to a pressing habitability dilemma.
Radio Signals from A Nearby Star
The small star YZ Ceti casts its dim reddish glow over a little collection of rocky exoplanets: three, and possibly a fourth, all of which orbit too close to the star to be habitable. YZ Ceti b, the closest of the group, takes just over two days to circle the star, and temperatures on its surface range from just below the boiling point of water to more than 400 degrees Fahrenheit. It’s not the sort of place you’d normally look for anything related to life or livable planets.
But when they observed the nearby star with the Karl G. Jansky Very Large Array, a radio telescope in New Mexico, Pineda and Villadsen noticed periodic flashes of radio waves coming from the system. No, it’s not aliens, but it could be evidence that YZ Ceti b has a magnetic field. If Pineda and Villadsen are correct — and they say they need more data before they can be sure—– the planet’s magnetic field interacts with the star’s magnetic field at certain points in the planet’s orbit, and those interactions produce radio waves.
Pineda and Villadsen published their findings in the journal Nature Astronomy.
A rocky, Earth-like exoplanet with a strong magnetic field of its own would be big news for habitability in general, despite YZ Ceti b’s scorching surface. If YZ Ceti b has a magnetic field, then so might other rocky exoplanets. And if that’s the case, then the odds of finding a rocky planet with an atmosphere in the habitable zone of a red dwarf just increased (even if we don’t know enough yet to quantify how much).
Habitable Zone, or Danger Zone?
If we’re going to find life, or even a livable world, orbiting another star, it’s probably going to be somewhere like TRAPPIST-1 or LP-890-9, also called SPECULOOS-2: rocky, Earth-sized worlds orbiting cool stars called red dwarfs.
TRAPPIST-1 is one of the current hotspots in the search for livable planets. The red dwarf star has three rocky exoplanets in the band of habitable space around it, out of seven total. But there’s a dark side to the star system; some studies have suggested that the red dwarf star might have stripped away the atmospheres of these worlds, leaving them airless and probably uninhabitable. Red dwarfs are known for producing violent stellar flares, and their habitable zones are situated very close by.
If so, that wouldn’t bode well for our chances of finding alien life, or even a clearly habitable planet.
Last week, a team of astronomers announced that TRAPPIST-1 b, the innermost world of the TRAPPIST-1 system, shows no signs of having an atmosphere — or at least, not more than a faint wisp of one. That’s not a complete surprise, given how close TRAPPIST-1b is to its star (it’s basically the TRAPPIST-1 version of Mercury, so it gets bombarded with more radiation and stellar wind than its planetary siblings), but it’s still discouraging.
“Had TRAPPIST-1b shown clear signs of an atmosphere in our observations, I would have been more optimistic for the cooler planets,” NASA astronomer Taylor Bell, a co-author of the TRAPPIST-1b study, tells Inverse.
Cornell University astronomer Nikole Lewis and her team will point JWST’s powerful instruments at TRAPPIST-1e, one of the system’s three habitable zone planets, this summer.
Planetary Force Field
But a magnetic field can offer a shield against charged particles that might otherwise steal away a planet’s atmosphere.
“Having an organized magnetic field (like Earth’s) could make it more likely that a planet could retain its atmosphere,” NASA astrophysicist Thomas Greene, who wasn’t involved in the YZ Ceti study, tells Inverse. “Charged particles from the star (analogs to our own solar storms) could be slowed or stopped by a field like that.”
Even at our relatively temperate distance from the Sun, Earth’s magnetic field plays a key role in keeping our planet livable. Earth has a fairly strong magnetic field thanks to our spinning core of molten metal, which basically turns the planet into a giant electromagnet. Mercury, surprisingly, also has a molten core and a magnetic field. On the other hand, Venus has no magnetic fields because their interiors have long since cooled and solidified.
The upshot of all that is that astronomers don’t actually know how common it is for a rocky planet to maintain a magnetic field at all — let alone long enough to support life. We just don’t have enough examples.
And that’s what’s so important about YZ Ceti b. Not only is it one more example, it also shows astronomers what to look for — the radio bursts — to find other rocky worlds that might have magnetic fields.
“For our understanding of how often planets can power and sustain, for billions of years, a strong magnetic field, it would be a big leap forward if we can confirm YZ Ceti b's magnetic field and start learning from thousands of exoplanets instead of being limited to a handful of Solar System objects,” Remo Burn, of the Max Planck Institute for Astronomy, who was not involved in the YZ Ceti study, tells Inverse.
Here’s the Catch
YZ Ceti b’s magnetic field interacts with its star’s magnetic field to produce radio waves, but that’s only possible because the planet orbits so daringly close to the star. Planets farther from their stars, such as those in the habitable zones of red dwarf stars like TRAPPIST-1, SPECULOOS-2, and others are too far away from their stars to generate radio bursts like the ones Pineda and Villadsen saw from YZ Ceti. In other words, astronomers won’t be able to look for radio bursts to find potentially-habitable planets with magnetic fields. But it’s still useful:
“Learning how common magnetic fields are would allow us to apply the knowledge to more temperate worlds and estimate how strong their magnetic fields are,” says Burn.
Meanwhile, Pineda and Villadsen plan to spend more time observing YZ Ceti. At the moment, they can’t completely rule out the idea that the radio bursts they saw with Very Large Array weren’t fueled entirely by the star itself, with no help from a planet’s internal electromagnet.
And even if YZ Ceti b — or any other planet — has a magnetic field, that’s no guarantee that it will have an atmosphere. That depends on how much mass the planet has, how thick its atmosphere is, and how strong the magnetic field is, not to mention how often, and how strongly, the star flares.
It’s even possible that the powerful flares from a star like TRAPPIST-1 could actually heat a planet’s interior enough to keep it geologically active — thus powering a protective magnetic field. We just don’t know yet. As always, time, and more data, will tell. But while a magnetic field doesn’t guarantee habitability, it’s likely that if we ever find a habitable planet, its atmosphere will be protected by a strong magnetic field.
“We think it is key for life to have an atmosphere which is neither too heavy nor too light. However, exactly those atmospheres would be most effectively lost without the protection of a magnetic field,” says Burn. “This makes it such a key ingredient for life as we know it.”