Planets outside our solar system are called exoplanets or extrasolar planets. For centuries, scientists, philosophers, and science fiction writers believed they existed, but there was no way of knowing.

When was the first planet outside our solar system discovered?

The first confirmed detection of planets outside our solar system came in 1992 when several terrestrial mass planets were discovered orbiting a pulsar. The outstanding discovery of a realm like our own was in 1995 when two scientists found 51 Pegasi b orbiting a sun like our own. As of May 2020, there are over 4,000 confirmed exoplanets and over 3,000 systems with almost 700 of them having more than one planet. The nearest exoplanet discovered, Proxima Centauri b, is located four light-years from Earth orbiting the closest star to the Sun.

How to find a planet outside our solar system

There are many methods of detecting planets outside our solar system, but Doppler spectroscopy and transit photometry have found the most. Astronomers can detect exoplanets indirectly by measuring their gravitational influence on the motion of their host star. The star will kind of look like it’s wobbly. More extrasolar planets were later detected by observing the variation of the star’s apparent luminosity as an orbiting planet transited in front of it. About 97% of all confirmed exoplanets have been discovered using indirect techniques like this. Almost all the planets detected so far are in the Milky Way. There is however evidence that extra galactic planets do exist; researchers in 2018 found them in a distant galaxy. There are approximately 2,000 extra galactic planets for every one star beyond the Milky Way.

Most known planets outside our solar system orbit stars roughly similar to the Sun. Some have been found orbiting binary star systems. Only a few planets in triple star systems are known and there is one in a quadruple system. Planets may form within a few years to a tens of millions of years after their parent star. When planets form in a gaseous protoplanetary disk, they create hydrogen helium envelopes. These envelopes cool and contract over time and depending on the mass of the planet some or all of the hydrogen and helium is eventually lost to space. An example is Kepler 51b, which has only about twice the mass of Earth but is almost the size of Saturn, which is 100 times the mass of Earth.

The different types of exoplanets

There are many different types of exoplanets, but today we will only be covering three: Hot Jupiters, Super-Earths, and Rogue Exoplanets. Hot Jupiters are a gas giant extremely close to their star. Some complete a single orbit, what would be like their year, in as little as a few days here on Earth. Astronomers were surprised by these hot Jupiters, because there is a planetary formation that indicates that giant planets should only form at large distances from their star, but eventually more planets of their sorts were found and it is now clear that hot Jupiters make up the minority of exoplanets.

Super-Earths are one of the most common types of exoplanets discovered so far with a mass between that of Earth and Neptune. The properties of these planets are largely unknown. If Super-Earths have more than 80 times as much water as Earth does, then they become an ocean planet with all land completely submerged.

Some exoplanets are so far away from their star that it’s difficult to tell whether they are gravitationally bound to it. Unlike Earth, most of the exoplanets don’t have a strong relationship with their significant star, so they are actually just wandering through space or loosely orbiting between stars. Then, there are the complete bachelors called Rogue Exoplanets who do not orbit any star at all. The rogue exoplanets in the Milky Way possibly number in the billions or more.

Are planets outside our solar system habitable?

One of NASA’s ultimate goals in the exoplanet program is to find unmistakable evidence of current life. In January 2020, scientists announced the discovery of the first Earth-sized planet in the habitable zone detected by tests. There is special interest in capturing evidence of a distant hospitable world where it’s possible for liquid water, a prerequisite for life here on Earth to exist on the surface. About one in five Sun-like stars have an Earth-sized planet in the habitable zone. This confirmed planets like ours can exist elsewhere in the universe. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in our galaxy alone. As four planets are discovered, we will ultimately tackle the prospect of life on planets beyond the solar system. Then, there’s life as we don’t know it. While it makes sense to search for something like ourselves first, we don’t yet know if that’s really what should be expected.