The age of exoplanets began in 1992, when astronomers detected a pair of planets orbiting a pulsar. Then, in 1995, astronomers discovered the first exoplanet orbiting a main sequence star. As NASA’s Kepler and TESS missions got going, the number of confirmed exoplanets continued to rise.
By 2015, NASA announced that Kepler had discovered its 1000th exoplanet. 2016 was a banner year for exoplanet detections with nearly 1500 in that year alone. The total number reached 5000 in March of 2022. Now, NASA has announced that there are 6,000 confirmed exoplanets.
Exoplanet science is, obviously, about more than sheer numbers. The variety of planets we’ve discovered teaches us vital things about nature, our own Solar System, and about the precious Earth. Curiously, many of the planets we’ve discovered are unlike anything in our own Solar System.
There are hot Jupiters, massive gas giants that orbit their stars in a matter of days. There are ultra-short period planets that put Mercury’s short orbital period to shame by completing orbits in mere hours. One strange type of planet are so close to their stars that they’re tidally locked to their star like the Moon is to Earth. These planets have one scorching hot side and one frigid side. Some of them may be hot enough to remain molten.
Others have such extreme temperatures, pressures, and chemical constituents that they may rain iron, or may be no denser than styrofoam. Some could be covered in oceans. Others swathed in toxic gases.
Somehow they’re all part of nature. Determining how they came to be is an enduring fascination.
But at the base of all of this searching and wondering is the one big question: Are we alone?
“Each of the different types of planets we discover gives us information about the conditions under which planets can form and, ultimately, how common planets like Earth might be, and where we should be looking for them,” said Dawn Gelino, head of NASA’s Exoplanet Exploration Program (ExEP) at the agency’s Jet Propulsion Laboratory in Southern California. “If we want to find out if we’re alone in the universe, all of this knowledge is essential.”
The vast majority of exoplanet detections are indirect. The transit method detects planets by measuring how much light an exoplanet blocks when it passes in front of its star. The radial velocity method detects the slight tugs exoplanets give to their stars and measure how the star’s light changes by wobbling. Astrometry detects miniscule movements, and in gravitational lensing, the presence of a planet introduces anomalies into the observed light. Both Kepler and TESS used the transit method, and that method is responsible for most exoplanet detections with almost 4500. Radial velocity is next with about 1140 detections.
Though effective, they’re indirect. Only direct imaging can measure the chemistry of exoplanet atmospheres and doesn’t require particular orbital alignments or orientations. But it’s difficult, and fewer than 100 exoplanets have been directly imaged.
6,000 confirmed exoplanets is a definite, concrete scientific milestone. But there are thousands of other candidates, and it takes a lot of work to confirm a candidate. Something else could be creating the signal, like stellar flaring or artifacts with the transit method. Follow up observations, sometimes with a different telescope, confirms them, and that takes a lot of time and observing resources. As of July 2025, TESS had a list of 7655 exoplanet candidates of which just over 600 have been confirmed.
“We really need the whole community working together if we want to maximize our investments in these missions that are churning out exoplanets candidates,” said Aurora Kesseli, the deputy science lead for the NASA Exoplanet Archive at IPAC. “A big part of what we do at NExScI is build tools that help the community go out and turn candidate planets into confirmed planets.”
We could be facing a glut of exoplanet discoveries that was unimaginable a couple of decades ago.
Candidate exoplanets are still being found in Gaia data, even though that mission ended. NASA’s Nancy Grace Roman Space Telescope, which should launch in 2027 unless the current administration’s threats to cancel it come true, should discover thousands more through microlensing.
The age of exoplanets is beginning to shift, though. Our searches are becoming more targeted. Rather than casting a wide net and seeing what they catch, astronomers are seeking to find more specific types of exoplanets. ESA’s PLATO is poised to detect many more rocky exoplanets around Sun-like stars after its launched in 2026. The Habitable Worlds Observatory is just a proposal at this point, but it will search for habitable exoplanets in habitable zones and will also contribute to the ballooning list of exoplanets. Other missions, like CHEOPS and ARIEL will study known exoplanets in greater detail.
The holy grail in exoplanet science is habitability. A lot goes into determining habitability, with only a few exoplanets displaying any possibility of being habitable. The key is finding biosignatures, particular chemicals that tell us life is active on a planet. The JWST with its infrared atmospheric spectrometry, is just beginning to address this and has already produced some tantalizing results, though nothing concrete yet.
Like all scientific endeavours, the search for exoplanets has been boosted by technological advances, and that will continue in the future. One of the big obstacles in exoplanet science concerns that stars that planets orbit. Stars are extraordinarily bright and the presence of a comparatively dim exoplanet can be entirely obfuscated by starlight. This is especially true in focused searchers for Earth-like worlds around Sun-like stars like the Habitable Worlds Observatory (HWO) is conceived to detect.
The HWO will need a powerful coronagraph or starshade to do its job. If a distant astronomer were searching for Earth around the Sun, they would have a tough time detecting it in all that starlight. That’s effectively what astronomers will be doing with the HWO.
China is beginning to leverage its technological prowess in the exoplanet sphere, too. Its Earth 2.0 (ET) Space Telescope is set for launch in 2028 and will spend four years searching for exoplanet transits. It’s China’s first dedicated exoplanet-hunting mission and is focused on Earth-sized exoplanets.
Eventually, we will have a list of confirmed Earth-like exoplanets around Sun-like stars. Then we’ll face an even more challenging task: figuring out if any of those worlds actually host life.
The original version of this article was published on Universe Today.
