From a planetary perspective, there's no place like home -- as far as we know, at least. Earth has everything we need to thrive: a breathable atmosphere, a plentiful water supply and a temperate climate. But that hasn't stopped us from looking for other intriguing interstellar planets, some of which we might even colonize in the distant future. Finding these planets, known as exoplanets since they're located outside of our solar system, is difficult work.
In January 2012, we knew of more than 700 confirmed exoplanets, quite a small number considering the billions of stars in our galaxy alone. Part of the difficulty is that we can only observe exoplanets indirectly, inferring their existence by the way they affect the stars they orbit. Still, astronomers are becoming more adept at finding exoplanets all the time.
We'll journey farther and farther away from our solar system to learn about some of the most amazing exoplanet finds.
As much fun as it is to imagine exploring other worlds spread throughout the galaxy, we all recognize how difficult it will be to travel such incredible distances. So it only makes sense that our first interstellar journeys head toward exoplanets as close to Earth as possible. At only 10 light-years away, Epsilon Eridani b is a great place to start. The only downside to that itinerary is that it's an extremely unlikely candidate for hosting life.
Not only is the planet a gas giant similar to our own Jupiter or Saturn, but it might follow an orbit that brings it close to and far from its parent star. If this is right, it means that even if the planet has terrestrial moons, they likely wouldn't be able to stand the extreme temperature swings that go along with Epsilon Eridani b's eccentric orbit. But some scientists argue the planet has a more regular, circular orbit instead, which makes the faraway system appear more like our own -- asteroid belt and all [source: NASA].
Actually, Epsilon Eridani is surrounded by two asteroid belts -- a possible sign that other, more Earth-like planets could be nearby on the inside of that belt, just like Earth is in our solar system.
When searching for Earth-like planets, astronomers look for what are known as Goldilocks planets. These planets aren't so far away from their parent star that they're frozen wastelands yet not so near that water boils off the surface. Unfortunately, Gliese 876d isn't a Goldilocks planet. In fact, the planet is nearly 50 times closer to its parent star (Gliese 876) than Earth is to the sun, and it may have a scorching surface temperature of 642 kelvins (almost 700 degrees Fahrenheit or 369 degrees Celsius) [sources: BBC, EPE].
But while Gliese 876d likely won't ever be home to humans, it does hold the distinction of being one of the first rocky super-Earths ever discovered. Thought to have a mass about 7.5 times that of Earth's, Gliese 876d offered in 2005 perhaps the first evidence that planets like our own are out there [source: BBC]. In fact, scientists typically call an exoplanet with a mass that's up to 10 times greater than Earth a super-Earth. Once the mass goes beyond that point, you start to get into gas giant territory.
More encouraging still is the fact that Gliese 876d is only 15 light-years away, proving that Earth-like planets might not only exist, but exist fairly close to home [source: BBC].
When scientists first discovered Gliese 581c, a confirmed super-Earth, in 2007, they thought the planet was, just possibly, capable of hosting life. Up to that point, Gliese 581c was one of the smallest exoplanets ever found, meaning it was likely rocky rather than gaseous. What's more, its orbit was right on the cusp of its star's habitable zone, meaning the planet might have liquid water. Scientists were so enthusiastic about the possibility of life on the planet that they beamed photos and text messages its way in 2008 [source: Empsak].
Unfortunately, subsequent research has tempered the initial excitement over Gliese 581c. We now know that the planet's orbit is slightly outside of the habitable zone and that, as a result, Gliese 581c's surface is likely too warm to have liquid water. That means we'll have to put any distant plans to colonize the planet that's 20.5 years away from our own on hold. That may be for the best, though; a year on Gliese 581c lasts just 13 days, so we'd have to spend a fortune on calendars [source: Sample].
If you're fascinated by the prospect of other life in the universe, look no further than GJ667Cc, an exoplanet confirmed in 2012. This planet is heralded as the best new candidate for life because it orbits comfortably inside the habitable zone of its host star.
As we mentioned, the perfect place for human colonization would have Goldilocks qualities -- features that aren't too cold for liquid water but not too hot to boil the stuff away either. GJ667Cc meets these standards and then some. It's about five times bigger than Earth and takes 28 days or so to orbit a star that's much dimmer than our sun. Two other stars exist in this exoplanet's system, but they're farther away -- at a distance similar to Saturn and Pluto from Earth.
We'll have to find out more before giving an exoplanet that's 22 light-years away an Earth-like reputation.
In order to study exoplanets, astronomers have had to be quite inventive. Exoplanets are simply too small, dark and distant for our telescopes to view them directly, so astronomers have found ways to observe their effect on other stars. For instance, to detect the super-Earth HD 40307 b, scientists at the La Silla Observatory of Chile observed the "wobble" that the planet caused in the star it orbits. This common method in the planet-hunting world, called radial velocity, requires extremely precise measurements to detect a planet's gravitational pull on a nearby star.
But while the method can verify a planet's existence, it leaves us with a lot of questions. For instance, although astronomers can calculate HD 40307 b's mass -- about 4.2 times the size of Earth -- they aren't certain whether HD 40307 b is gaseous or rocky [source: Barnes et al.].
Why would we even bring up the planet if we know so little about it? For one thing, at 41 light-years away, it's much closer to Earth than most exoplanets [source: Barnes et al.]. For another, two other super-Earths are in the same solar system, giving us plenty to explore once we arrive.
Liquid water would be perhaps the strongest sign that a planet could support life, and super-Earth GJ 1214b might have it in spades. In fact, astronomers think the planet might be one giant, deep ocean. Though in this case, the ocean might be too warm for it to support life.
Besides potentially having a watery surface, the planet is interesting for another reason. GJ 1214b is located a relatively close 41 light-years from Earth and orbits its star in a way that affords us a better view [source: Keim]. Why is this important? Both of these conditions make GJ 1214b ideal for observation. As the precision of our instruments improve, we should be able to learn some incredible information about the planet's atmosphere and composition, and, by extension, the nature of other solar systems.
A planet with two suns may seem straight from science fiction, like Luke Skywalker's home planet of Tatooine in the Star Wars saga. But in 2011, scientists found proof of Kepler-16b, the first definitive example of a circumbinary planet, or one that orbits two stars.
Since the Kepler-16 system is binary (it has two stars), scientists can pick up on regular dips in light emitted from the system every time the two stars eclipsed each other. But when they noticed other dips in brightness not caused by the eclipses, they knew that a third body was circling the stars. Lo and behold, it was Kepler-16b, an exoplanet about the size of Saturn.
But before you get your hopes set on a two-starred sci-fi paradise to colonize, beware. Kepler-16-b is gaseous, cold and orbits outside its system's habitable zone. It might take quite a bit of technology and equipment to make the exoplanet appealing to us warm-blooded humans.
If you like it hot, you might want to check out Kepler-10c. This scorching-hot exoplanet blazed its way to being officially acknowledged by scientists in 2011. It's a little more than two times the size of Earth and orbits its host star every 45 days [source: NASA]. Kepler-10c was first spotted by the Kepler space telescope some 560 light-years away from Earth. But what makes this distant sphere unique is how researchers confirmed its existence.
Scientists wanted to make sure the finding was a planet and not something else. By using a combination of tools, astronomers did just that. The space agency's Spitzer Space Telescope, in tandem with a new software called "Blender," provided the evidence needed to grant Kepler-10c planet status. The technique "blends" light from other sources around the potential planet and tracks them over time to ensure there's no mistake. In fact, the method allows scientists to be more than 99 percent sure that they're observing a planet and not some other celestial body.
Before 1991, scientists hadn't discovered a single exoplanet. Now, in addition to the hundreds of exoplanets discovered throughout the Milky Way, they've found six orbiting just one star: Kepler-11.
The Kepler-11 solar system is interesting not just because it has more planets than any we've observed outside our own, but also because it's extremely compact. For instance, five of the six planets orbit closer to their parent star than any planets in our solar system orbit the sun.
Kepler-11f stands out from the other planets in the system for its size, which is about 2.3 times the mass of Earth and makes it yet another super-Earth [source: NASA]. Like all of the planets in the system, Kepler-11f has a lower density than Earth, meaning its composition is likely quite different as well.
MOA-2007-BLG-192-Lb, or more succinctly MOA-192b, may not have the catchiest name in the galaxy, but it's a fascinating planet nonetheless. For instance, it has a mass only three times greater than Earth's, making it one of the smallest exoplanets ever discovered (and yet another super-Earth) [source: Kerr].
The planet orbits a star much smaller than the sun -- a star so small, in fact, that it can't sustain fusion reactions -- which is why astronomers believe MOA-192b is an icy, gassy planet like Neptune rather than a temperate planet like Earth. And while the planet is likely too chilly to make a good home, it gives us hope that smaller, Earth-sized planets may be abundant elsewhere in the cosmos.
Why are missions like Cassini important? Learn more about the Cassini mission in this HowStuffWorks article.
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