If you’re old enough to remember a world without texting, you probably also remember a time before we had definitive proof that other worlds existed outside our solar system. Though we’ve been searching the skies for other planets for just about as long as we’ve been exploring this one, it wasn’t until relatively recently that we found evidence of their existence.
Twenty five years ago last week, a paper was published in the journal Nature titled, “A planetary system around the millisecond pulsar PSR1257 + 12”. The paper marked the first discovery of planets outside of our own solar system.
An artist’s impression of what the first three exoplanets ever discovered might look like. Credit: NASA/JPL-Caltech
The first three planets discovered were utterly unsuitable to life, orbiting a cold, dense, radiation-spewing and swiftly spinning object called a pulsar. Even so, the planets served as faint signals of other worlds that might be out there.
It was an important moment in the search for other worlds. Alex Wolszczan, one of the authors of the original Nature paper writes, “From the very start, the existence of such a system carried with it a prediction that planets around other stars must be common, and that they may exist in a wide variety of architectures, which would be impossible to anticipate on the basis of our knowledge of the Solar System alone.”
Soon enough, more planetary systems started popping up. A few years later, in 1995, the first exoplanet orbiting a star similar to our own warm sun was discovered. The exoplanet, named 51 Pegasi B, is a hulking gas giant like Jupiter that’s orbiting 51 Pegasi, a star 50.9 light years away from Earth. With that, the race was on to find more exoplanets in the universe.
Space-based telescopes such as Kepler have helped discover many of these planets. Kepler has confirmed the existence of over 2,300 planets and found over 4,000 possible planetary candidates. The planets were initially identified by their transits–small dips in a star’s brightness that occur when a planet crosses between the star and Earth.
In addition to the sheer volume of exoplanets recorded by Kepler, the Hubble in particular, launched in 1990, has been instrumental in figuring out the atmospheric composition of many of these planets–ranging from water vapor to more noxious gasses. It found thick hydrogen atmospheres on two Earth-sized planets and clouds on Jupiter-like planets.
Some atmospheres are wilder than we could ever imagine, like the planet spotted by the Kepler space telescope with clouds having the same composition as rubies. Other exoplanets are exciting simply because they seem so normal. In 2015, researchers discovered the first Earth-sized planet in the habitable zone of a nearby star. That doesn’t mean that the planet is habitable (more information is needed), but it does mean that habitable worlds might exist.
The first Earth-sized planet confirmed to exist in a star’s habitable zone. Credit: NASA Ames/SETI Institute/JPL-Caltech
We now know that there are other worlds and planetary systems like ours and that they come in all shapes and sizes, from small rocky worlds like Earth to massive giants even larger than Jupiter.
A system of exoplanets located 2,000 light-years away from Earth, Kelpler-11 is one of the more complete planetary systems found. Credit: NASA/Tim Pyle
NASA’s Transiting Exoplanet Survey Satellite or TESS is set to launch next year and will be able to peer even deeper into the reaches of the universe. Other planet-hunting telescopes, both space and ground based, will continue the search for other worlds, including some that might resemble Earth. In addition to TESS, the James Webb Space Telescope is expected to launch in October 2018 and will look for exoplanets.
In just 25 years, we’ve gone from knowing only about the planets in our solar system and three small planets around a dim pulsar to gathering information on thousands of planets around thousands of stars. Who knows what the next 25 years will bring?