This month marks the 20th anniversary of the first discovery of a planet orbiting a Sun-like star outside of our solar system – 51 Pegasi b. This event represented a watershed moment in astronomy, and since this point, over 1,800 exoplanets have been discovered, with over 1,000 spotted by NASA's Kepler space telescope.
The study of exoplanets has revolutionized our understanding of the laws governing the greater mechanisms at work throughout the cosmos. The observations have allowed us to gain an appreciation of how unique our own solar system is, while granting us a glimpse at the incredibly varied nature of our galaxy.
We understand that our planet exists at the ideal distance from the Sun for the evolution of life, and that by searching for exoplanets that share similar characteristics with Earth, we can determine the habitability of planets orbiting distant stars.
The technological advances that have taken place since the discovery of 51 Pegasi b on October 6 1995 have allowed us to gain ever more complex insights regarding the nature of these remote worlds. As the methodology matures, we are learning to characterize the atmospheres present around them in ever greater detail, and from these observations, infer the prevailing surface conditions.
To celebrate two decades of otherworldly discovery, we've created a list of some of the weirdest, wondrous, and significant exoplanets identified to date.
A planet so large it shouldn't even exist. Imaginatively dubbed a "mega-Earth" by astronomers at the Harvard-Smithsonian Center for Astrophysics, the planet weighs in around 17 times the mass of our home world, and is so massive that it defies contemporary theories on planetary formation.
Beta Pictoris b
Beta Pictoris b was the first planet to have the length of its day measured. A full day on this massive planet lasts a mere eight Earth hours, as it spins at the ridiculous speed of 100,000 km/h (62,000 mph).
Exoplanet ring system orbiting J1407b
This selection isn't based on the planet itself, but rather the impressive ring system it hosts. The exoplanet with a mass somewhere between 10 - 40 times that of Jupiter is believed to host a ring system around 200 times larger than that of Saturn. The vast rings were discovered thanks to the pattern of unusual eclipses made as they passed in front of the exoplanet's young star J1407b.
HD 189733 b
This tidally locked exoplanet is basically a realtor's nightmare. Described as a "hot Jupiter", HD 189733 b sits only 4.65 million km (2.9 million miles) away from its parent star, giving it an average temperature of 800ºC (1,472ºF) and winds closing on 6,000 miles per hour (9,656 km/h). Oh, and also there's silicate snow showers, because space is fun.
51 Pegasi b
Another exoplanet categorized as a "hot Jupiter", 51 Pegasi b has two claims to fame. As previously noted, it was the first planet to be discovered around a Sun-like star, but after further observation from the High Accuracy Radial velocity Planet Searcher (HARPS) instrument mounted on the ESO's 3.6 meter telescope, it also became the first exoplanet to be observed in the visible light spectrum.
Described by NASA as "Earth's bigger, older cousin", Kepler-425b represents the most habitable exoplanet discovered to date. The planet, which is around 60 percent larger than Earth, orbits in its star's habitable zone, meaning that it is possible for liquid water to exist on its surface. The planet also shares a star with characteristics similar to our Sun, has a yearly cycle of 385 days and hosts a atmosphere at least as thick as Earth's.
It's tough keeping time on PH3c. The unusual orbit of the exoplanet causes extreme variations in the length of its yearly cycle, making the exoplanet impossible to detect via conventional methods. The cause of PH3c's eccentric orbit appears to be down to gravitational influence from nearby planets, resulting in a distortion in its orbital period of roughly 10.5 hours over the course of only 10 orbits.
Discovered in 2009, COROT-7b has a similar density and silicate rock makeup to that of Earth. One side of the exoplanet always faces towards its Sun, and this side is thought to have a temperature of 4220°F. Scientists have theorized that this all adds up to a planet where instead of raining water, it rains rocks!
These are just a tiny
sample of the plethora of diverse exoplanets discovered during the past two decades. We
can only imagine, with a new generation of planet hunting
observatories and techniques, what discoveries the next 20 years will