Artist illustration of NASA's Transiting Exoplanet Survey Satellite (TESS) in high-Earth orbit. (NASA)
Have you ever gazed up at the night sky and wondered which stars might have planets, what those worlds may be like, or if there could be some form of life on any of them? When I was a child, I did a lot of that sort of imagining — decades before the first scientific detection of an extrasolar planet.
We now live in an era of knowing that the galaxy teems with planets, and that probably most, if not all stars possess multiple worlds. Anyone born after 1992 has lived their entire life without needing to imagine if there are planets around other stars — we know they are there!
From that high orbit, TESS will engage in a two-year survey of 500,000 stars across the entire sky, searching for planets by the "transit" method: measuring the temporary dimming of a star's light when one of its planets passes in front of it.
What We Know About Exoplanets The search for extrasolar planets is not a new thing. We've been finding them since 1992, 26 years ago! As of April 2018, a total of 3,711 exoplanets of all sizes have been confirmed to exist. Their abundance tells us that most, if not all, stars in the galaxy likely possess at least one, and probably multiple, planets.
NASA's Kepler spacecraft, launched in 2009, set out to find the more elusive "Earth-like" exoplanets: world's close to Earth's size that could support liquid water on their surfaces, within their star's "Habitable Zone." Among the 2,600 exoplanets that Kepler has discovered, at least a couple dozen fall into this category.
Kepler's sampling suggests that there may be billions of these Earth-like worlds in the galaxy.
Naturally, scientists want to know more about these potential other-Earths. (So do I!) What are they made of? Do they have atmospheres? Do they have oceans? Most tantalizing of all, do they support life?
Unfortunately, most of the potentially Earth-like worlds we have discovered are too far away for us to learn much more than their sizes and how close they are to their stars. Their great distances from us make more detailed investigations extremely challenging, to say the least.
What's New About TESS?
Unlike the Kepler mission, which focused on very distant stars in one small patch of the sky, TESS will survey the nearest stars in our neighborhood of the galaxy, and across the entire sky.
Both of these factors will make detailed investigation by other observatories and spacecraft possible — including the upcoming James Webb Space Telescope, which will be tasked with measuring the temperature and atmospheric composition of these nearby worlds.
This may tell us if a planet has the necessary ingredients for life--liquid water and organic compounds. We might even detect the chemical telltales of life itself.
How Strange Might Strange New Worlds Be? As that child gazing up at the starry skies, I imagined some pretty wild possibilities for those yet-undiscovered worlds.
Imagine a planet-wide desert, stretching pole to pole, that is so cold that carbon dioxide lies frozen on the ground. Or a searing hot landscape with a corrosive atmosphere that is so thick it would crush you like an aluminum can. Or a cloud-darkened milieu where the rain, rivers and seas are cryogenic liquid methane and you would weigh only 20 pounds. Or a world covered entirely by a hundred-mile-deep ocean hiding under a crust of ice.
And these are only descriptions of some of the planets and moons in our own solar system.
TESS is projected to find at least 1,500 exoplanets orbiting nearby stars, and of these at least 300 are expected to be near-Earth sized. Once we begin to probe the environmental conditions on those planets, imagine what we might find.