Over the past three decades, astronomers have discovered planets orbiting sun-like stars in the universe. The discovery ended 2,500 years of debate about whether worlds outside our solar system existed, but it came with a shock. The most common type of planet in the universe is the kind of world that doesn’t exist in our little corner of the cosmos: what astronomers call “Super-Earths” and “Sub-Neptunes,” planets with much greater masses than our own that could theoretically support life.
Astronomers concluded a little over a decade ago that every star in the night sky hosts a family of worlds. Crucially for the search for life, one in five of those stars will have a planet in the “Goldilocks” zone (sometimes called the “habitable zone”). That’s the band of orbits that are just right for water to pool on a planet’s surface in puddles, lakes and oceans. It’s a good bet that life needs liquid water to exist, so the discovery of Goldilocks worlds (of which there are billions) has pushed the search for alien life to the top of science’s to-do list.
Are Super-Earths just enlarged versions of our world – or is something else entirely going on?
Planet hunting across interstellar distances is spectacularly difficult. At first, scientists could only learn the size and mass of newly discovered worlds from their telescopes. That was enough to see that the average galaxy planet—the kind most common in most solar systems—looks nothing like the worlds orbiting our sun.
Our solar system has two types of worlds. In the inner solar system there are “terrestrial” worlds – Mercury, Venus, Earth and Mars. These are basically spherical cakes of layered metal and rock; their cores are made mainly of iron and nickel; and their outer layers are made of rock – silicon, oxygen and magnesium.
Farther out in the solar system lie the “giants.” There are the gas giants Jupiter and Saturn, which are more than 100 times the mass of Earth and are made mostly of hydrogen and helium. Then come the “ice giants” Uranus and Neptune, which are surrounded by layers of frozen slush of ammonia, methane and other compounds.
Terrestrial worlds and giants. That’s it for our solar system. There’s no planet orbiting our sun with a mass between that of the terrestrial Earth and the ice giant Uranus (which is 14 times as massive as Earth). In the wider universe, however, the most common type of planet lies in that empty space between these two forms. These worlds are known as super-Earths and sub-Neptunes.
With billions of these planets scattered across the Milky Way, their ability to host life becomes an urgent question. Are Super-Earths simply scaled-up versions of our world that require scaled-up versions of Earthly life? Or are they something else entirely?
Answering this question has put scientists under pressure. This isn’t a metaphor—understanding Super-Earths and Sub-Neptunes means understanding pressure. A planet’s gravity increases with depth. At Earth’s core, the gravitational pressure is a million times that of the surface. Those high pressures deep inside our planet are no small matter. They shape the conditions for life on the surface, including the presence of tectonic plates, which shaped evolution, and magnetic fields, which protect us from dangerous solar radiation. What happens below shapes what’s possible above. All that extra mass in Super-Earths and Sub-Neptunes means their internal pressures are pushed to extremes that scientists don’t understand.
How does rock behave deep inside a Super-Earth when it’s being crushed 100 times harder than Earth? Will it still flow slowly, like asphalt on a hot day, creating something akin to Earth’s floating continents? Or are plate tectonics a scientific impossibility on Super-Earths? How does ice behave when subjected to the insane pressures deep inside a Sub-Neptune?
Directly probing the extreme pressures of Super-Earths and Sub-Neptunes is pushing scientists to their own extremes. Traditional laboratory methods can’t reach pressures 100 times greater than the Earth’s core, so we can’t replicate what’s happening on the surface of these distant worlds. To get to that limit, planetary scientists have teamed up with plasma physicists using giant lasers at places like the Laboratory for Laser Energetics in upstate New York (where I’m a scientist). These city-block-sized lasers were originally developed to study nuclear fusion.
The work has already borne fruit. High-intensity laser light was recently used to compress iron to Super-Earth pressures, giving us a clue to the temperature at which solid iron at the center of a Super-Earth melts into liquid form. Swirling currents of liquid iron at the center of the Earth are what give our planet its magnetic field—a field that deflects harmful particles from the Sun. The test suggests that Super-Earths may have their own life-saving magnetic fields.
K2-18b is a massive planet with eight times the mass of Earth and its ocean would be just as impressive
The extreme inner regions of Super-Earths are only half the problem for understanding these mysterious worlds. If we want to know more about the surface conditions where life can arise, astronomers must also understand their atmospheres.
Earth probably didn’t start out with the thin but life-giving veil of gases we enjoy today. When it first formed, it had a molten rock surface (a “magma ocean”) that was probably surrounded by a thick atmosphere of hydrogen and helium gases. But those elements are so light that Earth’s gravity couldn’t hold them for long, and they quickly bounced off into space. Only later, as the magma ocean cooled, were gases like carbon dioxide and nitrogen pushed out of the newly solidified rock to form a “secondary atmosphere.” (The oxygen we breathe came billions of years later.)
Super-Earths and Sub-Neptunes probably started out the same way. But because they are so much bigger, it is possible that they have retained their original atmospheres. When it comes to life on these worlds, everything depends on whether or not they have that.
Unlike our planet, most Super-Earths should have enough gravity to hold on to their big, bulging, ancient hydrogen atmospheres. Here’s the key point: Hydrogen is a powerful greenhouse gas, much better at trapping solar energy than, say, carbon dioxide. Hydrogen atmospheres are so good at trapping heat that the surfaces of these worlds can get hotter than a pizza oven. Not a great place for life. But astronomers have observed that many Super-Earth-mass planets don’t seem to have big, bulging, extended atmospheres.
So where did the hydrogen go? The culprit appears to be the host star, whose radiation can strip a Super-Earth of its hydrogen veil. This would indicate that Super-Earths are habitable. If they can shed secondary atmospheres like Earth did billions of years ago, their surfaces might become suitable for life.
Sub-Neptunes pose more atmospheric mysteries. Cambridge astronomer Nikku Madhusudhan and his colleagues recently made the radical proposal that some sub-Neptunes could be “Hycean Worlds.” Madhusudhan gave this name to planets with hydrogen atmospheres and vast oceans of liquid water on their surfaces (Hycean Ocean).
Madhusudhan realized that the enhanced greenhouse warming from hydrogen atmospheres could change everything we expect about habitable zones. By capturing so much of the sun’s energy in a hydrogen layer, planets far from their host star could still exist in a warm liquid state. If so, Hycean Worlds would be an entirely new class of habitable planets.
Using the James Webb Space Telescope, Madhusudhan’s team recently found evidence that K2-18b, a sub-Neptune orbiting a red dwarf star in the Leo constellation, showed strong evidence of a Hycean World. It’s a massive planet with eight times the mass of Earth, and its ocean would be just as impressive. Earth’s seas don’t go deeper than seven miles, but if K2-18b is Hycean, its oceans would cover the entire planet and reach depths of 500 miles. If 500-mile-deep Hycean World oceans exist, the question arises: Could life exist in dark seas where the pressure would be millions of times greater than the Earth’s ocean floor?
No one knows. Some of the questions about Super-Earths and Sub-Neptunes may require the next generation of giant ground-based telescopes, which are now being built. These monsters will have mirrors nearly as big as football fields. The effort will eventually pay off. Super-Earths and Sub-Neptunes are the strangest of all alien worlds because, until recently, we didn’t even know they existed. After thousands of years of pondering our solar system, we are suddenly confronted with how limited our vision has been. But these worlds will soon become familiar, and if we’re not alone, their inhabitants will be, too.
Adam Frank is a professor of astrophysics at the University of Rochester and author of A Brief Guide to Aliens.
