But that’s okay; the most likely world for life may not be like Earth, after all.
“You can spend too much time wondering which of identical twins is the more alike.” –Robert Brault
Of all the places we’ve ever looked in the Universe, only Earth has provided us with evidence that life exists there. But why is that? Is it because life is rare, and requires all of the conditions we find on Earth to bring it about and sustain it? Or is life ubiquitous, found in a great diversity of situations, and have we only found it here because this was the easiest place to find it? Because things have worked out well here, we tend to assume that if we have a planet and star with the same properties as the Earth and the Sun — the same age, the same orbital distances, the same sizes and masses, and made of the same materials — we’ll get life out again. We also assume that other combinations are less likely. Yet all of this may be a flawed set of assumptions, and Earth may be the cosmic rarity as far as life is concerned.
In 2015, NASA announced the discovery of Kepler-452b, and touted it as the “most Earth-like exoplanet” ever discovered. Sure, there were a lot of things it had in common with the Earth, and there was a lot its parent star had in common with the Sun:
- Its parent star is very much like the Sun in terms of temperature, mass and size: it’s a G2 star, with nearly the same brightness and the same overall lifetime.
- It orbits at almost the exact same distance and with a nearly identical period to our planet around the Sun: 385 days instead of 365.
- The star it orbits is only slightly more evolved than our Sun: older by 1.5 billion years, and hence it’s a little (20%) more energetic and a little (10%) hotter.
- The planet itself is slightly bigger than our Earth, with a radius about 60% larger.
Although this might possess the most overall “Earth-like” conditions of anything we’ve yet discovered, this world is certainly nothing like Earth.
In our own Solar System, the difference between Earth and Venus is tiny: right around 5% in terms of radius. But to step up, the difference between Earth and Uranus/Neptune is huge: these worlds are some four times the radius of Earth! So 60% larger might not seem like a whole lot, but it’s likely enough to push it over the edge from a rocky planet with a thin atmosphere to one that starts to have properties of a gas giant: a large envelope of light atmospheric gases. In fact, there’s a very narrow window you have to be considered “Earth-like” in terms of planetary size, and a deviation of more than 10–20% from Earth’s size is likely too much.
But there are reasons to be optimistic that Earth-like worlds are common. The latest results from Kepler indicate that there are at minimum some 17 billion Earth-sized planets just in the disk of the Milky Way: around at least a few percent of stars with at least one Earth-sized world. While the ultimate goal is to find a world with advanced biological life on it — a world with life at the time of the Cambrian explosion would still be amazing — our thoughts always return to Earth’s twin. Yet that might not even be the best place to look, even if it existed.
Our Sun is a 4.6 billion year-old G-class star. While you might look at the diagram above and think this makes us an “ordinary” star, the fact of the matter is our star is more massive than 95% of all stars out there! M-dwarfs, the little red guys all the way on the end, are by far the most common star type in the Universe, with three-out-of-every four stars being M-stars. In addition, our oceans will boil after another billion years or so, but M-stars burn at a stable temperature for up to tens of trillions of years!
Kepler has found plenty of Earth-like planets around these M-stars in terms of being in the right places for liquid water on their surface, and being of the right mass-and-size to be more “Earth-like” than anything else. While M-stars may experience flares more frequently, and their planets need to be closer in to their habitable zones (making tidal locking a greater possibility and flares more dangerous), they also offer more stable environments for their planets, with less UV radiation and with more protection from the random violence of interplantary/interstellar space. The tidal forces from their star are also stronger, and their short orbital periods give them an easy way to generate a large magnetic field, perhaps offering protection from flares and atmospheric stripping.
These systems are common; Earth-twin systems are comparatively much rarer. What would we need for a true “twin” of Earth? First off, we’ll need a star like the Sun. That means a star not only of both the same temperature and spectral class, but also of roughly the same age. It takes time for life to develop and evolve into something interesting, and that means we need a star system that’s at least many billions of years old. But we also can’t wait too long, because as stars age, the region of the core that fuses hydrogen into helium grows, meaning that power output (and brightness, and hence temperature) increases. Eventually, the planets (like Earth) that were once habitable will get too hot, permanently boiling the surface water and ending life-as-we-know-it.
So let’s say we’ve got about a 1–2 billion-year-window, or about 10% of the life of the star. There are some 200–400 billion stars in our galaxy, and about 7.6% of them are G-class stars, or the same type as our Sun. Even though our Sun is more accurately classified as a G2V star, that still means that around 10% of all G-class stars are the same type as our Sun. Estimating on the high end, that should tell you that there are 400 billion stars, 7.6% of which are G-class, about 10% of those are the same sub-class as our Sun, and about 10% of those are the right age to have interesting life, or some 300 million candidate stars. But even then, not all of them will have the right amount of heavy elements to produce an Earth-like world.
This is the spectrum of the Sun. Or, in other words, these lines you see are representative of all the different atoms — and their ratios — that come from the period table of elements. They’re abundant in our Sun, and they come in very specific ratios. The amount of everything that isn’t hydrogen or helium to all the fuse-able material in the Sun is what astronomers call metallicity. If we want an Earth-like world, we need a star with a Sun-like metallicity. This isn’t so bad; as many as 25% of the stars that were formed around the same time as our Sun were Intermediate Population I stars (like we are), and a great many of them (perhaps around 15% of those) have the same metallicity as our Sun, shown in green, below.
That means there are some 11 million stars in our galaxy that have the same type of home star we do, with the same abundance of heavy elements, that formed at the right time that they could have complex life on their worlds the same way Earth does. (And this doesn’t even take into account that many of the worlds with more or fewer metals could be more likely than Earth to have life. Like I said, just because it happened under our conditions does not mean our conditions are the most favorable for complex life to have happened!) So out of these 11 million solar “twins,” how many of them have Earth-twins in their habitable zones?
We need to form a rocky planet of the right size with the right elemental abundances, the right amount of water, and in the right location to be considered a twin of the Earth. These problems are all inter-related. You would think that if the central star has the right elemental abundances, then the planets it formed should have the same density-radius relationship as our Solar System does. But when you’re more than about 20% larger in radius than Earth, you’re very likely to have an envelope of the lightest gases in the Universe — hydrogen and helium — held onto by your planet’s gravity, even if you’re in the inner Solar System. One of the things we’ve learned from Kepler is that gas giants and super-Earths are common in the inner parts of star systems around other stars; we’re the anomaly.
A world that’s 60% larger than Earth would have about five times the mass, and that’s far too great to be rocky with a thin atmosphere, even in the inner solar system. There are, realistically when we run all the estimates, likely to between forty thousand and maybe a hundred thousand of real Earth-like planets in Earth-like orbits around Sun-like stars. Out of 400 billion stars, those are terribly restrictive odds.
And remember, the real goal of seeking these planets is to find worlds capable of harboring Earth-like life. If that’s the goal, don’t look for a “twin” of Earth; we’re better off looking at the smaller, Earth-sized worlds around M-class stars. We’re better off looking at Earth-sized worlds in the potentially habitable zones of their stars. The best bet for that is not to look for Earth-like orbits around Sun-like stars, but Earth-like planets in the right orbits around their stars. Of those possibilities, there are likely billions. That’s how we’ll get there.
We all want to find another world with complex life on it, and learn that, at last, we’re not alone in the Universe. What we need to find is the full suite of the right conditions, and to have a string of events give rise to the complex life we’re seeking. We must realize that while these conditions likely do occur on identical Earth-twins, that isn’t the most common place to find them. Our distant, dwarf-like cousins — the stars nothing like our Sun — might hold the keys to our most dearly-held dreams in the end.