When compared to Earth, planets like Venus and Jupiter are kind of weird. Oppressively hot or with blasting frigid winds of 380 mph, they’re not friendly places for humans. Using satellites and robotic rovers, we’ve learned quite a few details about the various planets orbiting our Sun. But what about other stars? What are their planets like? How weird do they get?
It turns out, pretty weird. And the way scientists have learned about the strange worlds out in the galaxy is fairly interesting--you might even say ingenious.
Artist’s impression of the extremely hot exoplanet Kepler-10b, nicknamed “Vulcan.” Credit: NASA
Looking for exoplanets (extra-solar planets, or planets outside of our solar system) is not as simple as getting a good telescope and pointing it to the stars. Planets don’t generate their own light, they only reflect the light of their star in the same way that Earth’s Moon reflects sunlight. That makes exoplanets very faint and nearly impossible to see. What’s more, planets are much smaller than their stars and so they tend to get lost in the bright glare of starlight.
But therein lies the key! Starlight itself carries quite a bit of information, if we know how to decode it. Without actually seeing the exoplanets themselves, we can still figure out that they exist. As an exoplanet orbits around its star, the planet’s path may take it directly between that star and the Earth. So for an observer on Earth, the exoplanet will cross in front of the star and will block a small amount of its light. While we can’t see the planet’s tiny shadow on the star’s face, we can measure that the brightness of the star has dropped slightly. If we watch the same star for a long time, and we notice that its brightness drops the same amount at a regular interval of time, we can infer that there is a planet orbiting that star.
NASA has entire space missions devoted to using this technique, called the transit method, to find exoplanets around thousands of stars in our galaxy. The Kepler Mission is a satellite that has been searching for exoplanets since 2009, and its data has confirmed the existence of more than 2,300 thus far! Other missions and techniques have brought the total confirmed exoplanets up to more than 3,600 and counting. NASA is now preparing to launch the Transiting Exoplanet Survey Satellite (TESS), which will be able to search even more of the sky for the subtle hints of planets transiting their stars.
Illustration of the Kepler spacecraft, which is currently in space looking for signs of exoplanets. Credit: NASA
There is a huge variety of sizes, types, and characteristics among the more than three thousand exoplanets we’ve discovered so far. For example, exoplanet Kepler-10b orbits its star so closely, zipping around in just 20 hours, that its surface is 2,500 degrees and covered in molten lava. On the opposite end of the temperature scale there’s OGLE-2005-BLG-390Lb, a cold and icy planet where the average temperature is a very chilly -364 degrees Fahrenheit. It is nicknamed “Hoth” after the fictional frozen planet from Star Wars. And because truth is stranger than fiction, another Star Wars-like planet (Kepler-16b, nicknamed “Tatooine”) orbits a binary star and so it has two suns. With so many planetary systems out there, it seems like almost anything is possible.
NASA's Kepler mission has discovered a world where two suns set over the horizon instead of just one. The planet, called Kepler-16b, is the most "Tatooine-like" planet yet found in our galaxy. Tatooine is the name of Luke Skywalker's home world in the science fiction movie Star Wars. In this case, the planet is not thought to be habitable. It is a cold world, with a gaseous surface, but like Tatooine, it circles two stars. The largest of the two stars, a K dwarf, is about 69 percent the mass of our sun, and the smallest, a red dwarf, is about 20 percent the sun's mass.
Most of what we know about the size of stars comes from pairs of stars that are oriented toward Earth in such a way that they are seen to eclipse each other. These star pairs are called eclipsing binaries. In addition, virtually all that we know about the size of planets around other stars comes from their transits across their stars. The Kepler-16 system combines the best of both worlds with planetary transits across an eclipsing binary system. This makes Kepler-16b one of the best-measured planets outside our solar system.
Kepler-16 orbits a slowly rotating K-dwarf that is, nevertheless, very active with numerous star spots. Its other parent star is a small red dwarf. The planetary orbital plane is aligned within half a degree of the stellar binary orbital plane. All these features combine to make Kepler-16 of major interest to studies of planet formation as well as astrophysics.
Credit: NASA/JPL-Caltech/R. Hurt
Imagine the weirdest possible exoplanet you can think of, maybe one where it rains sapphires or has oceans of bright purple goo. Then check every so often to see what new exoplanets have been announced. Maybe someday we’ll find an exoplanet like the one you thought up!