When people talk about finding life among the stars, it’s often a fairy tale that comes to mind.
That’s because we on Earth really hope to find a planet like ours in what’s called the Goldilocks zone—a region not too close and not too far from the star, but at just the right distance to support liquid water on a planet’s surface.
On Earth, water is arguably the most important ingredient for life, so it makes sense that we tend to think of it as a necessary criterion for habitable planets.
Still [as is often the case in cutting-edge science] there’s always the chance that we’re wrong, and looking for life *exactly* as we know it may be limiting the search.
In a new paper accepted for publication in the Astrophysical Journal Letters, planetary scientists Raymond Pierrehumbert and Eric Gaidos describe how life might evolve on a world outside the traditional habitable zone, thanks to a thick hydrogen atmosphere creating an unusual version of the planet-warming greenhouse effect.
Recently Eric was kind enough to walk me through the science and implications of a hydrogen greenhouse:
We’re not used to thinking of hydrogen as a greenhouse gas. Does it act as one on Earth?
No, molecular hydrogen (H2) is extremely scarce in Earth’s atmosphere, and even if it were abundant, there isn’t enough pressure to cause it to absorb infrared light, which is what a good greenhouse gas does.
How might hydrogen trap heat on an alien planet?
A greenhouse gas works by absorbing infrared light (what we feel as heat) and stopping it from escaping easily to space. Infrared light is absorbed by molecules when it causes them to bend or rotate, somewhat like how a passing wave distorts a string of floats on a line. Molecular hydrogen is a very simple molecule: It is composed of two atoms, and all alone it isn’t affected by infrared light [as if it's transparent to infrared]. However, in hydrogen gas at pressures several times what we experience at sea level, the molecules collide with each other so intensely that they began to change shape, and it begins to become opaque to infrared light (heat).
As an analogy, consider a thin sheet of plastic wrap. Stretched out, it is completely transparent; crumpled up, it is difficult to see through.
What types of stars did you examine in your study, and how do they compare to our sun?
We considered (hypothetical) planets around stars very similar to the sun, and other planets around stars about half the size of the sun and much less bright, called “M,” or red, dwarfs.
What is the classical definition of the habitable zone? Which planets in our solar system, for example, are in this zone? Does the zone change around other types of stars?
One reasonable definition of the habitable zone is this: Suppose we were able to move the Earth from its present orbit. As we moved it inward toward the sun, the surface would become warmer and water would evaporate into the atmosphere. Water is an excellent greenhouse gas, and this in turn increases the temperature further. As one moves the Earth closer and it becomes warmer, a point is reached at which the evaporation of water becomes unstoppable and the Earth’s surface essentially becomes a very hot sauna. This marks the inner edge of the habitable zone.
Now suppose we move the Earth farther from the sun: The Earth would grow colder, and glaciers would grow. Ice reflects more sunlight to space, so this causes the Earth to become colder still. Far enough from the sun, the North and South Poles become so cold that carbon dioxide from the air begins to freeze out into “dry ice.” This is a serious matter, because that turns off the greenhouse effect that carbon dioxide provides. Temperatures plummet, the oceans freeze over, and the Earth enters a permanent “snowball” condition. This is the outer edge of the habitable zone.
In our solar system only the Earth—and possibly Mars—are in the habitable zone. Mars has some other “problems,” such as no volcanic activity, that make it a comparatively inhospitable place.
Everything I said depends on the light from the sun that warms the Earth. Around a dimmer star, the habitable zone is closer, much as you must move closer to a dying campfire to keep warm. Around a brighter star, the habitable zone is farther away.
Illustration courtesy NASA
How far could a hydrogen-greenhouse planet be from its star and still be habitable?
Around a star like the sun, at least ten times the distance that the Earth is from the sun. Around a dimmer M star, about one and a half times.
What types of life might exist on a hydrogen-greenhouse world? How would gravity, atmospheric pressure, and light levels differ from Earth’s?
We don’t speculate, but we do consider different bacteria that make a living by harvesting sunlight just as plants do [called cyanobacteria].
If you were able to visit such a world, you would want to remain in your vehicle, because the pressure—perhaps 40 times that on Earth—would cause “nitrogen narcosis” and then, more seriously “oxygen toxicity.”
What would the landscape of such a planet look like, if a human were standing on the surface?
The sky would look red, and the light would be much dimmer, because only such red light could pass through the thicker atmosphere without being reflected around and back to space by the air molecules. (That reflection is what makes our sky blue.)
Of the 500+ known exoplanets, are there any that might be hydrogen-greenhouse planets?
Earthsize planets that are sufficiently far from their star might benefit from having a hydrogen greenhouse. But we don’t know if they actually have one or not. Two years ago, NASA launched a satellite called Kepler, which is finding hundreds of candidate planets around other stars. It does this by looking for the small dip in starlight that occurs when a planet fortuitously passes between us and its star. As the Kepler mission continues, it will find planets farther and farther out from their stars, and around the M stars I mentioned previously those planets will be very cold—except if they have a hydrogen greenhouse.
Could Saturn’s moon Titan have been a hydrogen-greenhouse world in the past?
That’s a good question and fun to speculate about. I doubt it. Titan is a big moon, but it would make a small planet, and its low gravity is not good at capturing enough hydrogen from the disk from which the solar system formed nor at preventing it from escaping to space. Titan’s hydrogen is instead formed when ultraviolet light breaks apart methane molecules.