Our solar system is peculiar.
Yes, there are strange worlds out there: moons harboring oceans, a desert orb that once teemed with water, and, of course, a planet brimming with strange, tentacled life. Yet our cosmic neighborhood is also unusual for what it doesn't have.
It's a golden age in the discovery of worlds beyond our solar system, called exoplanets. NASA has confirmed well over 5,000 of these planets. Among the most prevalent is a class of worlds dubbed "super-Earths." They are worlds ranging from some 30 to 70 percent bigger than Earth. They can be rocky (like Earth) or largely composed of thick, swirling gases. Or both. Around one-third of exoplanets discovered so far are super-Earths, meaning they're awfully common in other solar systems.
The back-of-the-envelope math is compelling. There are likely over a trillion exoplanets in our Milky Way galaxy alone. So as far as we know, the universe must teem with super-Earths — and some of them may be habitable, meaning they harbor conditions that could sustain life, if it exists there.
"They are indeed very exciting planets," Renyu Hu, an exoplanet researcher at NASA's Jet Propulsion Laboratory, told Mashable.
In 2022, for example, NASA announced the discovery of planet LP 890-9 c. It checks a lot of boxes for potential habitability. It's rocky, about 40 percent larger than Earth, and orbits in the "habitable zone" of its solar system, meaning a region where liquid water could exist, though it may be a cooler world than Earth. But what it's like there remains largely elusive.
What's in its atmosphere? Are any super-Earths truly like Earth? "We don't know a lot about super-Earths, because we don't have one in our solar system," Chris Impey, a professor of astronomy at the University of Arizona, told Mashable.
"They are indeed very exciting planets."
Another super-Earth discovered in 2022 may contain an ocean many times the size of Earth's. Fortunately, both new and forthcoming giant telescopes will enable scientists to peer into the atmospheres of these mysterious, distant worlds.
Revealing mysterious super-Earths
Finding new worlds is hard. Specialized telescopes like NASA's TESS space telescope must stare at stars and look for minute changes in their brightness. A star dimming might mean that a planet passed in front of the star, which could lead to the discovery of a new world. TESS has found over 240 confirmed planets so far, along with thousands of other candidates.
Once exoplanets are discovered, astronomers can look deeper. Today, the James Webb Space Telescope, the most powerful space telescope ever deployed, is equipped with instruments that can detect what exoplanet atmospheres are composed of. Earth, for example, contains bounties of nitrogen and oxygen, along with trace amounts of gases like carbon dioxide (though this carbon dioxide has a momentous influence on the climate).
But even the nearest planet is trillions of miles away. How can a telescope deduce what's happening on such a far-off super-Earth? Again, astronomers rely on starlight. When a planet transits in front of a star, light passes through the exoplanet's atmosphere, through space, and ultimately into instruments called spectrographs aboard the Webb telescope. They're essentially hi-tech prisms, which separate the light into a rainbow of colors. Here's the big trick: Certain molecules in the atmosphere absorb specific types, or colors, of light. So if that color doesn't show up in the spectrum of colors observed by a Webb spectrograph, that means it got absorbed by (or "consumed" by) the exoplanet's atmosphere. In other words, that element is present in that planet's skies.
Astronomers would like to point Webb at the super-Earth discussed above, LP 890-9 c. It holds promise to be a habitable world.
Importantly, Webb will get much-needed exoplanet-sleuthing help later this decade. Of note, the aptly named "Extremely Large Telescope" being built in the profoundly dark Chilean desert will collect light passing through exoplanet atmospheres, too. Its main mirror is over 127 feet across, allowing the instrument to even take pictures of some exoplanets. "The ELT will revolutionize the study of planets outside our solar system," writes the European Southern Observatory, Europe's astronomical collaboration in the Southern Hemisphere.
Though we'll learn bounties more about super-Earths in the coming decades, lots will remain elusive. We're inherently limited by our address in the cosmos. The Webb telescope, for example, can only view exoplanet atmospheres that it can see transiting in front of their stars. Webb needs the perfect angle to see this happen, but our telescopes aren't often at the right angle. A typical planet's transit around the most common type of star in our galaxy (called a red dwarf) has just a two-percent chance of being detectable, explained Ravi Kumar Kopparapu, an exoplanet researcher at NASA's Goddard Space Flight Center.
The closest planet to us, Proxima Centauri b, is a super-Earth discovered in 2016. Its existence is known from detecting the slight wobbles of its star, which are caused by Proxima Centauri b's pulls and tugs. But none of our current telescopes are positioned to capture light from its atmosphere. Much of the planet remains mysterious to us, even though it's profoundly close, cosmically-speaking.
"We can't know much about it until we have more powerful telescopes," Kopparapu told Mashable.
Life on habitable worlds?
Might some super-Earths truly be habitable, meaning life could potentially dwell there? "That is one of the fundamental questions we want to know," said Kopparapu.
But if a super-Earth is indeed habitable, that doesn't mean there's life there. Not nearly. "A habitable planet can be habitable but not inhabited," Kopparapu emphasized.
Impey, of the University of Arizona, suspects there are quite a few habitable super-Earths. Why? They have masses larger than Earth, giving them better odds at holding onto a thick atmosphere which protects them from harmful radiation and UV rays, he noted. Super-Earths may also hold onto bounties of water. "Water is not a rare ingredient in the cosmos," Impey added.
"Nature can fool us in many different ways."
NASA's Hu is especially interested in super-Earths that are "cold." This doesn't imply that they're snowballs. It means they're not scorched by close orbits around their star (many exoplanets are discovered close to their stars, like TRAPPIST-1b, which might be rocky, but circles its star in just 1.5 days). "There are a couple that likely have surface temperatures that are comparable to Earth," Hu said. "They offer the right physical conditions for us to search for potential habitability."
Conversely, some exoplanets may have steam atmospheres, explained Kopparapu. Torrid climes could have evaporated this water from the surface. Such a world is indeed hot, but perhaps life could dwell in some regions, or subterranean places, on or in the planet's rocky surface. After all, life on Earth thrives in sweltering environments around underwater volcanic vents, and in the thermal pools of Yellowstone National Park.
Actually identifying life on a distant super-Earth, however, is a different, profoundly challenging story. Indeed, we can journey to remote places on Earth and find life thriving in extreme places, such as organisms dwelling deep under Antarctic ice. "But for exoplanets we can't do that," said Kopparapu.
From trillions and trillions of miles away (many light-years), an abundance of evidence must come together to support any assertion that a super-Earth likely contains life. Context is key, explained Kopparapu. Is the planet exposed to dangerous radiation? Is there enough data, from telescopes, to create a computer simulation of what the planet's environment is like? Are there alternative ways, beyond life, to explain how certain gases, like methane, could have been produced?
"We have to be really careful," Kopparapu emphasized. "Nature can fool us in many different ways."
Even on the closest planet to Earth, Venus, there's been lively scientific debate in recent years about whether small amounts of a gas, phosphine, could be a hint of life in the Venusian atmosphere. Then in 2021, astronomers concluded the gas probably isn't even present on Venus. The evidence for extraterrestrial life is a hard sell, even in our own solar system.
In the search for habitability on super-Earths, astronomers may ultimately discover that few are actually like Earth. Maybe they're dominated by different gases, or don't have rocky lands jutting out from water oceans. And would that be such a bad thing?
"I would be disappointed if they're Earth-like," said Kopparapu. "We want to explore strange new worlds."
