Exoplanets, or planets outside of our solar system, have been highly sought after ever since humans first went to space. In 1992, we discovered two of them orbiting the pulsar PSR B1257+12. These qualify as planets, but not the way we generally think of them—they were most likely recondensed gas and dust, formed from the blasted remains of the planets that had been orbiting the pulsar before it exploded. It would be years before researchers found planets orbiting a sun-like star like ours. Now, the National Aeronautics and Space Administration has confirmed over 5,000 different worlds that exist outside of our solar system.
We're still a long way from being able to actually observe distant alien worlds up close—but we can confirm their presence and some of their characteristics. The first two exoplanets discovered around PSR B1257+12 were spotted because they impacted the way the star pulsed.
When a giant star reaches the end of its life, its core collapses and the star explodes outward, releasing all of its energy into the space around it. Some dead stars become black holes. Others become neutron stars. Pulsars are a specific type of neutron star that gives off radio frequency pulses at regular intervals, like a heartbeat. When PSR B1257+12 exhibited pulses that were just slightly off, scientists were intrigued. They then observed that the interval in which these pulses were irregular had its own regularity and arrived at the conclusion that this was most likely caused by two planets made of recondensed debris orbiting the pulsar.
While two planets made of the dust and gas of dead worlds may not sound very promising, the fact that they existed at all was cause for celebration. If planets could form around a pulsar, of all things, it meant that the creation of planets was much easier than previously thought. That, in turn, meant that there was very likely to be an enormous number of extrasolar planets out there.
After this, scientists focused on searching for planets star-by-star. The radial velocity method lets them detect exoplanets around stars with low mass, who more readily exhibit the gravitational effects of the planets around them. They appear to “wobble,” or move back and forth as their planets orbit. As they move toward and away from Earth, the light they emit changes slightly due to the Doppler effect.
Other methods involved direct imaging, or accidentally spotting planets as they passed in front of other observed objects. As of 2018, the radial velocity method, direct imaging, and accidental observation accounted for about 900 exoplanets. Then there's the transit method.
Compared to other ways of spotting exoplanets, the transit method has been the most successful. As planets orbit a star, they may end up coming between the star and the observer. Planets are a tiny fraction of the size of the stars they orbit, but this tiny, brief blockage dims the stars' light enough to be detectable. NASA’s Kepler spacecraft has spotted thousands of transiting objects, suddenly and dramatically expanding the roster of extrasolar planets. Ever since its launch in 2009, Kepler has contributed over 3,000 confirmed new exoplanets to the list. Another 3,000 await confirmation.
We have yet to leave our own solar system, so the information we can gain about exoplanets is somewhat limited. Nonetheless, there's a lot you can infer about a planet based on how it interacts with the objects around it. If you detect a wobble in a star, for example, and it's due to the orbit of a planet, you can use the mass of the star and the degree of the wobble to calculate the mass of the planet. If you know how bright a star is and spot a transiting planet passing in front of it, you can use this information to determine the size of that planet.
This has resulted in the discovery of some very fascinating alien worlds. Some are massive gas giants that orbit so closely around their stars, they reach temperatures hotter than some other stars. Some planets are rocky, like Earth, but many times our planet's size. Others are akin to our solar system's Neptune, with atmospheres dominated by helium or hydrogen.
Exoplanets are tremendously far away, and, so far, we can only learn about them based on how they interact with objects that are equally far away. While the information we can glean allows us to make certain inferences, there are still a lot of things we don't know. As technology advances, we may be able to detect an even wider variety of exoplanets in the future—including ones that may be able to support life.
