What Is an Exoplanet? NASA's TESS Mission Explores Alien Worlds

On Wednesday, NASA's newest space telescope TESS sprinted to orbit and within about two months it will begin identifying exoplanets. Exoplanets are what scientists call planets in other solar systems orbiting stars beyond our sun.

Astronomers have positively identified 3,717 exoplanets in less than three decades. But TESS, short for Transiting Exoplanet Survey Satellite, is designed to identify thousands more, opening our eyes even further to the diversity of worlds in our universe.

Exoplanets are much trickier to study than our own neigbors—even the nearest known exoplanet is still lightyears away. But even over that distance, scientists can figure out a lot of information about these alien worlds. The exoplanets they've seen so far are even wilder than the seven planets that neighbor us around the sun.

Read more: NASA's New Telescope Won't Be Able to Tell How Likely Alien Life Is on New Planets—This Is Why

Scientists have found an exoplanet covered in lava across the entire half that faces its sun, one hurtling toward Earth, one almost large enough to be a star, another with an upper atmosphere full of carbon monoxide, and one with winds that blow the wrong way. They've found an entire solar system with as many planets as our own, further destroying humanity's hope our solar system is somehow special.

But astronomers can't really take pictures of these worlds, which even the most powerful cameras struggle to photograph. Instead, they usually learn about these distant worlds indirectly. Planets are first spotted by how they impact their star—causing a slight dip in brightness as they pass between the star and a telescope or how the pull of their gravity makes stars appear to wobble.

Exoplanet Kepler-69c, as depicted by an artist. NASA/Ames/JPL-Caltech

Then information about the star itself and the planet's size and orbit let scientists begin to paint a mental image of these planets, what their surface may look like, and even whether they could support life.

TESS's predecessor, the Kepler space telescope, turned exoplanets from a rarity into something practically commonplace. At this point, scientists suspect that most stars in our sky host at least one planet.

That mindset is a huge shift, and one TESS will continue. But our incredible wealth of exoplanets is still new. Many of the leaders of the TESS mission and the field of exoplanet studies in general were trained in an era when we truly didn't know if our sun was uniquely well endowed.