In just over two decades, we’ve gone from knowing about nine planets (shut up, Pluto haters) to recognizing the existence of thousands of them. Almost all of those are completely unsuitable for life as we know it, but researchers say it’s only a matter of time before we find “Earth 2.0”. Like any other 2.0 version, I’m sure that one will have fewer features and be harder to use, but never mind that.
As so many science fiction writers like to add in a large helping of verisimilitude, I thought I’d write a post summarizing what we know about planets outside our solar system, or exoplanets, as we enter 2017. Keep in mind that there’s a lot even I don’t know, although I’ve been following the field as a lay observer since 2000. Nonetheless, I hope there’s enough in here to stimulate your imagination. Also, this will necessarily be a technical post, so fantasy authors beware.
What we know
We know planets exist beyond our solar system. They’ve been detected by the way they pull on their stars as they orbit (the Doppler or radial velocity method), and that’s how we found most of the early ones. The majority of those known today, thanks to the Kepler mission, have been discovered by searching for the change in their stars’ light intensity as the planets pass before them: the transit method. In addition, we have a few examples of microlensing, where the gravity of a planet bends the light of a “background” star ever so slightly. And we’ve got a handful of cases where we’ve directly imaged the planets themselves, though these tend to be very, very large planets, many times the size of Jupiter.
However we see them, we’re sure they’re out there. They can’t all be false positives. And thanks to Kepler, we’ve got enough data to start drawing some conclusions. Of course, these must be considered subject to change, but that’s the way of science.
First, our solar system, with its G-type star orbited by anywhere from eight to twenty planets (depending on who’s counting) starting at about 0.3 AU, looks very much like an outlier. We don’t have a “hot Jupiter”, a gas giant exceedingly close to the star, with an orbit on the order of days. Nor do we have a “warm Neptune” (a mid-range gaseous planet somewhere in the inner system) or a “super-Earth” (a larger terrestrial world, possibly with a thick atmosphere). This doesn’t mean we’re unique, though, only that we can’t assume our situation is the norm.
Second, we’ve got a pretty good idea about which stars have planets. To a first approximation, that’s all of them, but the reality is a little more nuanced. Bright giants don’t have time to form planets. Small red dwarfs don’t have the material to create Jupiter-size giants. Neither of these statements is an absolute—we’ve got examples of gas giants around M-class stars—but they’re tendencies. Everything else, seemingly, is up in the air.
What we can guess
Planets do appear to be everywhere we look. There are more of them around M stars, but that’s largely because there are so many more M stars to begin with. A lot of stars have planets with much closer orbits, so close that you wouldn’t expect them to form. Gas giants aren’t restricted to the outer system, like they are here. And there’s a whole class, the super-Earths, that we never knew existed.
We can make some educated guesses about some of these planets. For example, many of the super-Earths, according to computer simulations, may actually be tiny versions of Neptune, so-called “gas dwarfs”. If that’s true, it severely cuts our number of potentially habitable worlds. On the other hand, the definition of the habitable zone has only expanded since we started finding exoplanets. (Even in our own solar system, what once was merely Earth and maybe the Martian underground now includes Europe, Titan, Enceladus, Ganymede, Ceres, the cloud tops of Venus, and about a dozen more exotic locales.) Likewise, studies suggest that a tide-locked planet around a red dwarf star doesn’t have to be frozen on one side and scorched on the other.
We’ve got a few points where we don’t even have data, though. One of these, possibly the most important for a writer, is the frequency of Earthlike worlds. By “Earthlike”, I don’t simply mean terrestrial, but terrestrial and capable of having liquid water on the surface. Where’s the closest one of those? Until about a year ago, the answer might have been anywhere from 15 to 500 light-years away. But then came Proxima b. If it turns out to be potentially habitable—in the month and a half between my writing this post and it going up, we may very well know—then that almost ensures that Earthlike worlds are everywhere. Because what are the chances that the next-closest star to the Sun has one, too?
Creating a planet
For the speculative writer, this lack of knowledge is a boon. We have the freedom to create, and there are few definite boundaries. Want to put a gas giant in the center of a star’s habitable zone, with multiple Earthlike moons? We can’t prove it’s impossible, and the real-life counterpart might really be out there, waiting to be found.
Basically, here’s a rundown of some of the factors that go into creating an exoplanet:
Star size: Bigger stars are shorter-lived, but smaller ones require their “classically” habitable planets to be much closer, to the point where they’ll likely be tide-locked. G-type dwarfs like ours are a happy medium, but not a common one: something like 1% of stars are in the G class, and there’s not much data saying that planets are more likely around them.
Star number: Most stars, it seems, are in multiple systems. Binaries can host planets, though; we’ve detected a class of “Tatooine” planets (named after the one in Star Wars, because scientists are nerds) circling binary systems. For close binaries, this is a fairly stable arrangement, but with huge complexities in working out parameters like temperature. Distant binaries like Alpha Centauri can instead have individual planetary systems.
Planet size: We used to think there was a sharp cutoff between terrestrial and gaseous planets, based on the difference between the largest terrestrial we knew (Earth) and the smallest gas planets (Uranus and Neptune). Now we know that’s simply not true. It’s more of a continuum, and there may be super-Earths much larger than the smallest mini-Neptunes. And those gas dwarfs appear to be the most common type of planet, but that could be nothing more than observation bias, the way we thought hot Jupiters were incredibly common ten years ago. On the smaller end of the scale, we haven’t found much, but there’s no reason to expect that exoplanet analogues of Mars, Mercury, Pluto, and Ganymede don’t exist.
Surface temperature: This is a big one, as it’s critical for life as we know it. We know that liquid water exists between 0° and 100°C (32–212°F), with the upper bound being a bit fluid due to atmospheric pressure. That 100 (or 180) degrees is a lot of room to play with, but remember that it’s not all available. DNA, for example, can break down above about 50°C. Below freezing, of course, you get into subsurface oceans, which might be fun for exploration purposes.
Atmosphere: Except for a couple of gas giants, we’ve got nothing here. We have no idea if the nitrogen-oxygen mix of Earth is common, or if most planets we find would be CO2 pressure cookers like Venus. Or they could retain their primordial hydrogen-helium atmospheres, or be nearly airless like Mars. Something tells me that we’ll find all of those soon enough.
Life: And so we come to this. Life, we know, changes a planet, just as the planet changes it. A biosphere will be detectable, even from the distance of light-years. It will get noticed, once telescopes and instruments are sensitive enough to see it. And it will stand out. Some chemicals just don’t show up without life, or at least not in the quantities that it brings. Methane, O2, and a few others are considered likely biotic markers. The million-dollar question is just how likely life really is. Is it everywhere? Are there aliens on Proxima b right now? If so, are they single-celled, or possibly advanced enough to be looking back at us? Here is the writers’ playground.
What’s to come
Assuming the status quo—never a safe assumption—our capability for detecting and classifying exoplanets is only expected to increase in the coming years. But I’ve heard that one before. Once upon a time, the timeline looked like this: Kepler in 2004 or 2005, the Space Interferometry Mission (SIM) in 2009, and the Terrestrial Planet Finder (TPF) in 2012. In reality, we got Kepler in 2009 (it’s now busted and on a secondary mission). TPF was “indefinitely deferred”, and SIM was left to languish before being mercy-killed some years ago. The Europeans did no better; their Darwin mission suffered the same let’s-not-call-it-cancelled fate as TPF. Now, both missions might get launched in the 2030s…but they probably won’t.
On the bright side, we’ve got a small crop of upcoming developments. TESS (Transiting Exoplanet Sky Survey, I think) is slated to launch this year—I’ll believe it when I see it. The James Webb Space Telescope, the Hubble’s less-capable brother, might go up in 2018, but its schedule is going to be too crowded to allow it to do more than confirm detections made by other means.
Ground-based telescopes are about at their limit, but that hasn’t stopped us from trying. The E-ELT is expected to start operations in 2024, the Giant Magellan Telescope in 2025, and these are exoplanet-capable. The Thirty Meter Telescope was supposed to join them in about the same timeframe, but politically motivated protests stopped that plan, and the world is poorer for it.
Instead of focusing on the doom and gloom, though, let’s look on the bright side. Even with all the problems exoplanet research has faced, it’s made wonderful progress. When I was born, we didn’t know for sure if there were any planets outside our own solar system. Now, we’re finding them everywhere we look. They may not be the ones science fiction has trained us to imagine, but truth is always stranger than fiction. Forget about “Earth 2”. In a few years, we might have more Earths than we know what to do with. And wouldn’t that make a good story?