Of the thousands of exoplanets discovered so far, the most common are “super-Earths” – worlds larger than Earth but smaller than Neptune. Some of these planets lie even within the habitable zones of their parent stars, which means conditions may be right to host liquid water on their surfaces.
But can these giant, rocky exoplanets sustain conditions for life? Or is life restricted to smaller planets like ours? In the end, only future observations will give us a definitive answer. But in the meantime, the question gives us the perfect opportunity to explore the frontiers of our understanding of where life might find a home.
Related: What makes a planet truly habitable? Our assumptions may be wrong
First, we must be clear about what we mean by “habitable,” since some of the wild worlds in our solar system extend far beyond what we consider natural. After all, no other planet is quite similar a land.
little other, Rocky planets in our solar system are either barren lands (Mercury And the Mars) or Hell Nightmare (Venus). The gas giants – With its deep and charming atmosphere – is excluded. However, some of their frozen moons offer ample amounts of liquid water under their hard shells, and they might be a great second habitat for life in our own backyard. But for now, we’ll limit our discussion to Earth-like worlds.
This means that in our super-Earth survey, we need to find planets that look and act (and hopefully smell) a lot like our own. This includes sitting inside the star’s habitable zone, to make sure the temperatures are just right, as well as having a relatively thick but not too thick atmosphere. These planets also need to have liquid water on their surface, not trapped under a frozen crust or boiling away in steam. Finally, they must have a file magnetic fieldto protect this atmosphere and liquid water from the continuing brutal attack of solar wind.
Certainly, there are a lot of criteria that must be in place for a world to actually host life. But without these prerequisites, the chances of something growing in an alien world are pretty slim, so it’s a good place to start.
Choose the right planet size
Astronomers generally define a giant Earth as any planet that lies between Earth size and 10 times the mass. Astronomers tend to call planets bigger than that Mini NeptuneHowever, this seemingly clear distinction hides many nuances that are important for determining habitability.
Obviously, something closer to the size of Earth has a better chance of being habitable, because it is supposed to be very similar to Earth. And something closer to the size Neptune Perhaps it would not be a very interesting place for life to find a foothold, because Neptune in general is not all that hospitable, at least according to the definition above.
As the mass of the planets rises, the rocky core becomes better and better at attaching to a thicker atmosphere and gases, due to its strengthening gravity. In the end, there will be so much atmosphere that the planet can be better described as a gas giant than a rocky world. Unfortunately, we lack a clear dividing line between these two extremes, and super-Earths bridge that gap.
This is where orbit matters, too. If a planet is very close to its parent star, regardless of its size, it will be roasted. Takes 55 Cancri E, a giant rocky Earth about 55 light-years away. Its mass is eight times that of Earth, but it is so close to its parent star that it is just a ball of molten rock.
The planet TOI 270c, on the other hand, is smaller, with a mass about seven times that of Earth. But it’s so far from its parent star that it’s almost entirely gaseous, making it more like a small Neptune than anything else.
Ultimately, a super-habitable Earth should have the right density, indicating that it’s not too rocky or too gassy. Even at that time, this is just a guess, as astronomers have scant information about any particular thing extrasolar planet.
take this example, Gliss 581 EGP, which is only about 20 light-years away. This exoplanet has a mass about 5.5 times the mass of Earth and lies within the habitable zone of its star. But astronomers only know its mass, not its radius, so they can’t determine the planet’s density. In that orbit and that mass, the planet could be a typical rocky world, or made of solid iron. Or it could be a small gas world, or even made of diamonds.
As for the magnetic field of any exoplanet, this is a matter of pure guesswork. Scientists believe that planets larger than Earth likely host strong magnetic fields, but it’s impossible to know for sure. For example, while Venus and Earth are roughly the same size, only Earth has a large magnetic field.
Probably The best filter for super-habitable Earth is LHS 1140bthat revolve around a red dwarf star About 49 light years from Earth. It is about 60% wider than our planet but 6.48 times larger. It orbits very close to its parent star – its orbital period is only 25 days – but because the star is a cool red dwarf, this is comfortably within the habitable zone.
Atmospheric models for LHS 1140b allow for the possibility of a dense atmosphere embracing a world with oceans of liquid water. Only detailed notes, perhaps with James Webb Space Telescope, will tell us for sure if the planet is truly habitable. In the meantime, it is the current world champion for the largest planet that can host life.
Paul M Sutter He is an astrophysicist in State University of New York Stony Brook and Flatiron Institute, host”Ask an astronaut” And the “space radio“and author”How do you die in space?. Learn more by listening to the ‘Ask an Astronaut’ podcast, available at Itunes (Opens in a new tab) And the askaspaceman.com. Ask your own question on Twitter using #AskASpaceman or by following Paul Tweet embed And the facebook.com/PaulMattSutter.