The first major discovery of an “exoplanet” – a planet outside our solar system –      happened in 1992. Two astronomers found not just one but two planets orbiting not a sun but a pulsar 2,300 light years away. Pulsars, named for the regular “pulses” they give off in radio frequencies, are remnants of supernova explosions, when a massive star runs out of nuclear fuel and becomes unstable. A pulsar packs the mass of the entire sun into a ball only 12 miles (20 km) across, so dense that it can easily collapse and become a black hole. It spins around hundreds of times per second, spewing radio waves, x-rays, and radiation.

The pulses are so regular that if they don’t come at a precise interval, astronomers know something is off. This pulsar should have pulsed every 0.006219 seconds, but every now and then, its pulses were a little off. Yet those off-beats came at regular intervals. After intensive study, the answer was found: it had two planets in orbit around it. One was three and the other four times the mass of Earth, and they rotated around the pulsar every 67 and 98 days.

The next challenge was to find an exoplanet that revolved around a star like our own sun rather than a pulsar. In 1995 two astronomers were observing a sun similar to ours called 51 Pegasi using a spectrograph. They noticed a pronounced wobble in its orbit which could only be caused by the gravity of an orbiting planet. This planet turned out to be a gas giant about half the mass of our own Jupiter in an extremely close, four-day orbit around the star. A year on this planet lasts only four days.

But a veritable explosion of planetary discoveries came with the invention of the “transit method” of observation, when the Kepler Space Telescope launched in 2009.

The Kepler telescope constantly observed over 170,000 stars, searching for tiny dips in starlight caused when a planet crosses a star’s face. The greater the dimming, the bigger the planet. The Kepler telescope was replaced with the Transiting Exoplanet Survey Satellite (TESS) in 2018 which was 400 times larger.

By March of 2022, over 5,000 planets had been discovered in the Milky Way Galaxy ranging from lava worlds covered in molten seas, to puffy planets the density of Styrofoam. Of the 5,000 planets found so far, 30% are gas giants; 31% are rocky in nature; 35% are icy and usually the size of Neptune or Jupiter; and 4% are terrestrial in nature, about the size of Earth and rocky.

Then the next leap came when the first planet was identified in a different galaxy. In 2012, NASA’s Chandra Observatory spotted a curious flicker coming from the Whirlpool Galaxy, some 28 million light-years away. An X-ray source in one of Whirlpool’s arms switched off for about two hours before suddenly flaring back to life. This particular source emanated from two objects in orbit around each other. One is either a black hole or a neutron star, and the other is likely a large star known as a “blue supergiant.”

Aatronomers began to suspect the cause for the dimming may have been a planet about the size of Saturn that had briefly blocked X-rays from reaching the telescope when it passed in front of it. The team dubbed it an “extroplanet.” The extroplanet candidate goes by the name “M51-1” and is believed to orbit its host binary at about the same distance Uranus orbits our sun. It will be another 70 years before the planet crosses that path again, giving researchers another chance to observe the first planet ever found in another galaxy.