Why “new” planets may change our search for aliens

A team from Cambridge has defined a new class of potentially habitable planets.

Astronomers have discovered more than 4,000 planets around distant stars so far, but none of these exoplanets have shown any clear signs of hosting life.

But a new study published Wednesday in The Astrophysical Journal suggests one reason for the absence of aliens might be that we’ve been limiting ourselves to looking at planets too much like Earth.

And the authors have a suggestion of where to look: a class of planets acting a bit like a gas giant and a bit like a rocky one, with thick atmospheres hiding deep oceans.

By expanding our search to planets previously considered too large and too hot to host life, the researchers theorize we may well increase our chances of detecting biosignatures of aliens.

It’s a hypothesis that could change the way scientists hunt for alien life within a few years and might be the best shot we’ve ever had at answering one of the biggest questions in astronomy: Are we alone?

What’s new — Nikku Madhusudhan of the Cambridge Institute for Astronomy and his colleagues have identified a new class of exoplanets too large to have large rocky cores like Earth but smaller than a planet like Neptune.

These planets have a few unique characteristics:

  • They carry at least 10 percent of their mass in water and likely possess a global ocean
  • They’re swaddled in a hydrogen atmosphere
  • Their temperatures could veer between Earth-like climates and temperatures above 392 degrees Fahrenheit

The authors came up with a new term for such worlds: “hycean,” a portmanteau of “hydrogen” and “ocean.” Their unique characteristics could make them a prime place to hunt for life beyond Earth.

“I have to be careful, but my guess is that this is going to be a paradigm shift in the search for life on exoplanets,” Madhusudhan tells Inverse. “We hadn’t even considered planets like this to be good candidates for life.”

Why it matters — Astronomers primarily search for planets outside our solar system via the transit method, where they observe a planet pass in front of its star and measure how the star’s light dims.

A graphic illustration of the transit method for observing exoplanets around distant stars. NASA

The transit method can yield other important information, Madhusudhan says. When a planet passes in front of its star, so, too, does its atmosphere. Astronomers can look at the sunlight passing through the atmosphere, picking up the signatures of several chemicals, including biosignatures like oxygen, methane, or dimethyl sulfide.

There’s a hitch: finding out information from a transit is really hard to do for Earth-like exoplanets around Sun-like stars, Madhusudhan says. The smaller the planet and the larger the star, the harder the planet is to observe. “It’s all about contrast,” Madhusudhan says. “For an Earth-like planet around a Sun-like star, it’s exactly the worst combination. Your planet is the smallest, but the star is largest.”

Because of that difficulty, many astronomers searching for signs of atmospheres on exoplanets focus on Earth-like planets around smaller dwarf stars. But what Madhusudhan and his colleagues suggest is that scientists go bigger and start looking for life and much larger planets that will be easier to observe, like hycean planets.

Hycean worlds are not only easier to observe with a space telescope, Madhusudhan says — they are also more numerous. By expanding the search for signs of life beyond the more traditional Earth-like planets around small stars, we can increase the chances of picking up biosignatures.

Here’s the background — Madhusudhan began to suspect the definition of a habitable exoplanet could be expanded while studying an exoplanet named K2-18b, which orbits a red dwarf star 124 light years away from Earth. While observing the planet, which is between Earth and Neptune in mass, Madhusudhan realized such planets might be able to maintain liquid water at higher temperatures than on Earth.

An artist’s rendering of the exoplanet K2-18b.ESA/Hubble, M. Kornmesser

Typically when scientists calculate the habitability of an exoplanet, they don’t bother with planets much warmer than 395 Kelvin, or 251 Fahrenheit, “because long before that, you will evaporate the entire ocean,” Madhusudhan says.

But in the new study, Madhusudhan and his colleagues show that the large amounts of water on hycean worlds, along with their hydrogen-rich atmosphere, could allow liquid water to persist at such high temperatures — temperatures known to be survivable by extremophile bacteria here on Earth.

“We know life can survive until about 120 degrees Celcius and under pressures of around 1000 bar,” he says. “Let’s make that as our limiting case for habitability.”

It also opens up new definitions of the “habitability zone” around a star, or where its planets could host life. Take Earth and set it on an orbit much further out than 1.75 astronomical units — just past the orbit of Mars — and the greenhouse gases will freeze out, Madhusudhan says, and the planet goes cold.

Habitable hycean worlds, in contrast, could be found anywhere from as close to a Sun-like star as the orbit of Venus to as far out as, well, anywhere — even outside a star system.

“They can be free-floating planets,” Madhusudhan says, their insulating hydrogen atmospheres preserving habitable conditions even in the vast darkness of interstellar space.

Why it matters — Ultimately, the new study is just a hypothesis, and Madhusudhan is quick to caution anyone from interpreting the paper as suggesting they will find life on Hycean worlds.

“All we are saying is this is fundamentally a new opportunity in our search for life, and it allows us a testable hypothesis for whether life exists on such planets,'' he says.

And while complex on a rocky Earth-like planet remains the Holy Grail for exoplanet scientists, “we don’t have to sit waiting on it,” Madhusudhan says. “In the meanwhile, you could explore a whole volume of planets that are more observable, characterizable, and maybe you find life.”

What’s next? — Scientists will have to wait at least a little bit longer to study hycean worlds. To do the sorts of measurements necessary to detect biosignatures on such planets in a reasonable amount of observing time, you need a big telescope like the James Webb Space Telescope, which is currently scheduled to launch in November.

“Hubble is difficult,” Madhusudhan. “But with James Webb, you can do it relatively easily.”

So how soon will we, possibly, know if there are aliens in vast, hot ocean worlds?

“We are talking two to three years,” he says, “If all goes well.”

Abstract — We investigate a new class of habitable planets composed of water-rich interiors with massive oceans underlying H2-rich atmospheres, referred to here as Hycean worlds. With densities between those of rocky super-Earths and more extended mini-Neptunes, Hycean planets can be optimal candidates in the search for exoplanetary habitability and may be abundant in the exoplanet population. We investigate the bulk properties (masses, radii, and temperatures), potential for habitability, and observable biosignatures of Hycean planets. We show that Hycean planets can be significantly larger compared to previous considerations for habitable planets, with radii as large as 2.6 R⊕ (2.3 R⊕) for a mass of 10 M⊕ (5 M⊕). We construct the Hycean habitable zone (HZ), considering stellar hosts from late M to sun-like stars, and find it to be significantly wider than the terrestrial-like HZ. While the inner boundary of the Hycean HZ corresponds to equilibrium temperatures as high as ∼500 K for late M dwarfs, the outer boundary is unrestricted to arbitrarily large orbital separations. Our investigations include tidally locked ‘Dark Hycean’ worlds that permit habitable conditions only on their permanent nightsides and ‘Cold Hycean’ worlds that see negligible irradiation. Finally, we investigate the observability of possible biosignatures in Hycean atmospheres. We find that a number of trace terrestrial biomarkers which may be expected to be present in Hycean atmospheres would be readily detectable using modest observing time with the James Webb Space Telescope (JWST). We identify a sizable sample of nearby potential Hycean planets that can be ideal targets for such observations in search of exoplanetary biosignatures.