How This New Probe Will Hunt for 'Alien' Earths

Editor’s Note: This story was updated at 7:05 p.m. ET on April 18.

This evening, NASA’s next planet hunter took to the skies aboard a SpaceX Falcon 9 rocket launched from Cape Canaveral, Florida. After a two-day launch delay caused by issues with the rocket’s onboard guidance and navigation control systems, the Transiting Exoplanet Survey Satellite, or TESS, successfully blasted off.

“LIFTOFF! Our @NASA_TESS spacecraft has left the launch pad at 6:51pm ET to search for unknown worlds beyond our solar system! Watch #TESS live,” NASA announced on Twitter. About 10 minutes later, the first stage of the Falcon 9 rocket landed back on Earth, touching down on a drone ship off the Florida coast.

Barring any malfunctions, the TESS spacecraft will now settle into orbit and begin searching for planets around the nearest, brightest stars, building up our catalog of alien worlds that are close enough to be scrutinized for signs of life.

“A few months after TESS launches, we will be able to point out the first ones of these familiar stars, which host planets that could be like ours,” says Cornell University’s Lisa Kaltenegger.

If a planet-hunting spacecraft sounds familiar, it’s likely because of the rich diversity of worlds found so far by NASA’s venerable Kepler space probe. Since 2009, Kepler has been harvesting planets from the cosmos, spying the footprints of these alien worlds in distant starlight. Kepler alone can claim more than 2,600 discoveries, some of which could be rocky planets quite similar to Earth.

TESS won’t be doing exactly the same thing as Kepler, says mission scientist Elisa Quintana of NASA’s Goddard Space Flight Center. Here’s how the two missions stack up, and how TESS will help us probe the pervasive question of whether we’re alone in the universe.

Why do we need more than one planet-hunting eye in the sky?

After nine years in orbit, the Kepler spacecraft is running extremely low on fuel, and it will most likely cease operating within the next couple of months. It’s an inevitability that scientists have been preparing for, but that doesn’t make the spacecraft’s end any easier. No one is looking forward to losing what has been the most successful planet-finder in history—a mission that revealed there are more planets in the sky than stars.

Even if Kepler did have a longer life span, more exoplanets are always better. What’s more, TESS is looking for particular types of planet, rather than taking a broader census of what’s out there.

“I’m most excited about the large number of planets that are between the size of Earth and Neptune that TESS will find,” Quintana says. “These are the most abundant types of planets, and we know very little about them, because there are none in our solar system.”

So how will TESS find planets?

Like Kepler, TESS will be searching for planets that cross the faces of their stars from our perspective, causing their star’s light to dim briefly. Those tell-tale dips in brightness not only reveal the presence of a planet, but also how wide that world is and how long it takes to orbit its star.

Unlike Kepler, which stared at a single patch of starry sky for most of its primary mission, TESS will be looking all over. As it zooms around Earth, the spacecraft will be using four cameras to gaze at a whopping 85 percent of the sky, focusing on particular stars for 27 days at most. Its first year of observations will focus on the southern sky, and its next year will map the north.

“Before we knew what a huge number of fascinating, diverse planets were out there, the approach of scanning the sky and only looking at each star for a short time was too risky,” Kaltenegger says. “Now that Kepler showed us how many planets are out there, and also how many planets that could be like our own are out there, scanning the sky makes sense.”

Why do we care about these types of planets?

The search for life beyond Earth is necessarily constrained by what we know. Life as we don’t know it could be anywhere, and it doesn’t care that we haven’t deigned to imagine it yet. To help focus the hunt, astronomers are starting by looking for something familiar. And we know that at least once, life evolved on a warm, rocky planet orbiting a relatively stable star.

That being said, many of the stars TESS will scrutinize will be smaller and dimmer than our own: the cool, reddish M dwarfs that are the most common types of stars in the Milky Way. Planets orbiting these stars at a distance that’s neither too hot nor too cold for liquid water to exist are going to be snuggled in quite close—orbiting near enough to their stars for scientists to find them on months-long time scales.

In addition, the worlds TESS expects to find will be better situated for observations that could reveal whether alien metabolisms are churning away on their surfaces, beneath their seas, or in their clouds.

Wait, why will the TESS planets be easier to study for signs of life?

Within its field of view, TESS will be targeting the nearest, brightest stars to our own, which means every star you can see in the night sky, plus at least a few hundred thousand more. This observing scheme will allow scientists to find planets that are prime for follow-up observations: those that are both relatively nearby (within 300 light-years or so of Earth), and that circle stars bright enough to illuminate an alien atmosphere.

The majority of Kepler’s planets are too far away and circle stars that are too dim for these kinds of observations. (Here’s how aliens might detect signs of life on Earth.)

How does that work, exactly?

As these worlds migrate across their stars, background starlight will shine through any alien atmosphere that’s present, offering clues about its composition and puffiness—but it takes an extremely sensitive telescope to collect those clues over cosmic distances.

Assuming that the long-delayed James Webb Space Telescope eventually launches and is operational, it will be a leading actor in probing distant atmospheres and characterizing the worlds TESS reveals. In the meantime, other telescopes will be used to measure the masses of the TESS planets, which will help astronomers calculate their densities and compositions and determine when a planet transitions from rocky, like Earth, to something that’s gassy, like Neptune.

The Hubble and Spitzer space telescopes could even get in on some of the atmospheric action until Webb comes online, Quintana says.

“The delay of Webb is unfortunate,” she says, referring to the new launch estimate of 2019. “However, it does mean that there will be time for TESS to accomplish its prime mission and create a catalog of planets from both hemispheres.”