Our solar system, with its itty-bitty rocky planets on the inside and gaseous giants on the outer, is looking more and more like an oddball as astronomers continue to uncover scores of exoplanets.
Astronomers rarely find Venuses or Earth when they look closely at stars in faraway solar systems.
Rather, the common inner planet appears to be a cross between a rocky dwarf and a gaseous giant.
These omnipresent worlds have been called “mini-Neptunes” by researchers, who believe they are scaled-down analogs of our solar system’s ice giants.
The nature of mini-Neptunes has remained a mystery due to the lack of a local version to analyze.
However, their massive atmospheres press down with severe pressures, leading many academics to believe that these worlds are little more than barren hydrogen and helium balls.
Now, a group of exoplanet scientists claims that at least some mini-Neptunes may bear no resemblance to Neptune.
They discovered that the atmospheric pressures on some of these planets may allow hot-but-not-scalding oceans to endure for billions of years after rigorous modeling.
With that in mind, the team concludes that this common sort of inner planet may be home to extraterrestrial germs and that any lifeforms on these worlds would alter their atmospheres in ways that astronomers would soon be able to detect.
Nikku Madhusudhan, an astronomer at the University of Cambridge and the primary author of the new study, says, “[These findings] considerably expands our possibilities of finding life.”
A miniature Neptune has been discovered.
Astronomers can estimate an exoplanet’s mass and size based on how it interacts with its host star, giving them an approximate idea of whether it’s mostly rock or gas.
However, researchers are mostly blind when it comes to deciphering the finer nuances of what lies between a planet’s solid core and its atmosphere.
However, in 2019, a team of researchers used the Hubble Space Telescope to peer inside the atmosphere of K2-18b, a mini-Neptune.
They discovered traces of water vapor and clouds, prompting Madhusudhan and his colleagues to calculate what kinds of interiors were possible based on the new information about climatic conditions.
They reported this year that some of the conceivable forms of K2-18b could include planet-wide oceans that, despite the world’s heat, pressure, and size, would be tolerable to at least some Earth life.
“We established for the first time that planets can be far larger than Earth and still have viable conditions,” Madhusudhan explains.
A new world has opened up for you.
Many scientists’ greatest wish is to locate a perfect Earth twin with the same mass, size, and orbit—near enough to its star for water to stay moist but not so close that it boils away.
However, Madhusudhan wondered if the discipline should be thinking more broadly after learning that K2-18b (which compresses eight Earth masses into a volume nearly 17 times larger) could be drowning in water.
An Earth analog could be excellent for the complicated organisms that go on to build telescopes and the internet, but it’s overkill for the bacteria that live in hot springs and deep-sea vents.
“Let us not assume [human-friendly] conditions,” he says.
The researchers were able to identify one group of mini-Neptunes as habitable planets by abandoning established assumptions.
They discovered that a sufficiently thick and heavy hydrogen-rich atmosphere could operate like a pressure cooker lid, allowing all-encompassing seas to remain even in the extreme heat experienced by planets orbiting their host star.
Hycean worlds (pronounced HI-shun) is a combination of the words “hydrogen” and “ocean,” according to the researchers.
The researchers detailed a standard Hycean planet as well as two variations that all match their criterion of habitability (temperatures below 250 degrees Fahrenheit and pressures below 1,000 times that of Earth’s atmosphere).
The conventional Hycean planet would be all ocean with no continents, with fog and mist rising from the steaming seas.
“Dark Hycean” planets are those whose spin has been corrected by tidal forces so that one side of their planet always faces their star, similar to how the same side of our moon always faces the Earth.
The dayside would be hot, while the night side would be chilly.
Finally, even at large distances from their host star’s warmth, the hydrogen atmospheres of “Cold Hycean” worlds may trap enough heat to keep the planets’ waters from freezing.
The trio of Hycean worlds broadens the range of planets that could be considered habitable, continuing a recent trend in exoplanet research that seeks more sophisticated definitions of habitability.
We have this “idea of finding exactly Earth 2.0,” says Björn Benneke, a University of Montreal astronomer who was not involved in the study.
“There may be other means of finding other life besides dinosaurs crawling around on the surface,” says the researcher.
From candidates to approval
The Hycean hypothesis, unlike many other hypotheses in the inherently speculative realm of astrobiology, can be directly tested, and soon.
The galaxy is littered with mini-Neptunes.
While many of them will lack the pressures and temperatures required to form and maintain deep seas, their sheer numbers almost assure that some will.
“Even if 5% of them met this criteria,” says Benneke, who assisted in the discovery of water vapor in K2-atmosphere, 18b’s “that would still be a lot of planets.”
Because hycean prospects are so plentiful, astronomers may be picky, focusing on planets with the best viewing circumstances.
Madhusudhan and his colleagues discovered eleven prime targets in their paper, which was published yesterday in The Astrophysical Journal.
Astronomers dream of the kinds of atmospheres found on hycean worlds.
Researchers can figure out what’s in a planet’s atmosphere by observing it pass in front of its host star and seeing which wavelengths of light it blocks and which it lets through.
The atmosphere of an Earth-like planet is made up of heavy molecules like oxygen and nitrogen that cling to the planet, making it razor-thin and difficult to view.
However, because a Hycean planet is surrounded by light hydrogen and helium molecules, its atmosphere expands thousands of times faster than Earth’s.
Astronomers might be able to detect a handful of molecules produced almost solely by creatures on Earth using the future James Webb Space Telescope (JWST) in such bright atmospheres, according to Madhusudhan’s team.
I’m just waiting for it to be discovered.
If the JWST launch goes smoothly this fall, the first observations looking for indications of life in K2-atmosphere 18b’s might happen as soon as next year.
Madhusudhan believes that after just 20 hours of observation, the gleaming new space telescope will be able to spot one of the proposed “biosignatures” (assuming they exist).
And he’s one of several researchers who have already been granted permission to execute such a study during JWST’s first operational cycle.
Although a single finding would not prove that K2-18b is swimming with alien germs, it might encourage other astronomers to hunt for more biosignatures to support the argument.
“If that happens, the floodgates will open,” Madhusudhan adds.
“We could be looking at a two-to-three-year time window for biosignature detection.”
Benneke is preparing his life-hunting observations, based on two different lines of thought.
He plans to hunt for preliminary biosignatures in the atmosphere of one of the TRAPPIST-1 solar system’s stony, Earth-like planets.
He’ll also look at a mini-Neptune orbiting a faint star, which may be a Hycean world.
“Both of these [paths] have a lot of things to be enthused about,” he says. “This is a fantastic time to be alive.”
the author is :
Charlie Wood Charlie is a journalist covering developments in the physical sciences both on and off the planet. In addition to Popular Science, his work has appeared in Quanta Magazine, Scientific American, The Christian Science Monitor, and other publications. Previously, he taught physics and English in Mozambique and Japan, and studied physics at Brown University.
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