Exoplanets — Finding Worlds Around Other Stars
Are there planets beyond our Solar System? Since the first confirmed discovery in 1995, thousands of exoplanets have been found. Here's how they're detected, and what it would take for one to support life.
Worlds orbiting stars other than the Sun
An "exoplanet" is a planet orbiting a star other than our own Sun. For a long time, the existence of planets around other stars was pure speculation. Stars shine brilliantly on their own, but the planets orbiting them produce no light of their own and are vastly smaller, making them extraordinarily hard to spot directly against their star's glare.
That long-standing speculation became fact in 1995, when Swiss astronomers Michel Mayor and Didier Queloz confirmed a planet orbiting 51 Pegasi, a Sun-like star. The discovery earned them the 2019 Nobel Prize in Physics.
How they're found — wobbles and shadows
There are two main methods for finding exoplanets. The first is the "radial velocity method," which exploits the fact that a planet's gravity tugs its star into a tiny wobble. By precisely measuring the subtle shift in the star's light wavelength as it moves slightly toward and away from Earth (the Doppler effect), astronomers can infer a planet's presence.
The second is the "transit method," which catches the tiny dimming of a star's light as a planet passes between it and Earth. If that dimming repeats on a regular cycle, it's evidence of a planet. NASA's Kepler space telescope, launched in 2009, used this method to find thousands of exoplanets, driving an explosion in the field.
Where life could exist — the Goldilocks zone
Of particular interest are exoplanets with conditions that might support life. The distance from a star where surface temperatures allow water to exist as a liquid — neither frozen nor boiled away as vapor — is called the "habitable zone," or, borrowing from a children's story, the "Goldilocks zone."
Sitting in the Goldilocks zone doesn't automatically mean life exists there, though. Many more factors have to align — the presence and composition of an atmosphere, a magnetic field, the host star's activity — and confirming any actual signs of life requires analyzing the atmosphere directly.
The exoplanets found so far
Thousands of exoplanets have been confirmed to date, and they're far more varied than anyone imagined. Discoveries include "hot Jupiters" — Jupiter-sized worlds orbiting so close to their star they scorch — "super-Earths," rocky planets somewhat larger than our own, and even "rogue planets" that belong to no star at all, drifting alone through space.
The TRAPPIST-1 system, about 40 light-years from Earth, drew particular attention for hosting seven roughly Earth-sized rocky planets at once. More recently, the James Webb Space Telescope has begun directly analyzing the atmospheres of some exoplanets, pushing the search for signs of life into a new phase.