What are Exoplanets?:
Exoplanets are planets that exist outside of our solar system. Most of these planets orbit other stars but some may not be bound to any star at all. Such planets are called rogue planets.
The first exoplanets were discovered in the 1990s and to date, we have discovered 4000+ exoplanets using various methods and scientific instruments.
Classification of exoplanets:
So far, scientists have classificed exoplanets into four types:
- Gas Giants
Terrestrial planets are rocky planets like Earth or Venus. They are mostly made of rock or iron and have a solid or liquid surface and they may or may not be similar to Earth. To be called a terrestrial planet, their mass must range from 0.5 to 2 times the mass of the Earth.
To date, 163 Terrestrial planets have been discovered. Some notable terrestrial rocky planets are TRAPPIST-1 e and TRAPPIST-1 d. They orbit the same star. TRAPPIST-1 e is the 4th planet from its star and lies in the habitable zone. TRAPPIST-1 d is the 3rd planet from its star and has liquid water in the twilight zone (between day and night side).
Super-Earths are planets whose mass ranges from 2-10 times the mass of the Earth. The “Super-Earth” name is misleading to most. “Super-Earth” only refers to the mass of the planet and has nothing to do with their similarity to Earth (although they could be similar). There are no Super-Earths in our solar system.
To date, 1340 Super-Earth’s have been discovered in the Milky Way. Some notable super-Earths include GJ 15 A b (takes only 11 days to orbit its home star) and 55 Cancri e (located only 41 light-years from Earth).
These are planets like Jupiter and Saturn– mostly composed of hydrogen and helium gas. These planets don’t have a solid surface.
Gas Giants that orbit close to their home star are called Hot Jupiters. These gas planets are so close to their star that their temperature exceeds thousands of degrees. Some of these planets orbit the star in as little as 18 hours!
To date, 1359 Gas Giants have been discovered. A notable Gas Giant is Pegasi 51 b. It was the first exoplanet discovered to orbit around a sun-like star. Pegasi 51 b is located 51 light-years from Earth and is a “Hot Jupiter” (temperatures range from 538 to 982 degree C). The discoverers of the planet, Michael Mayor and were awarded the 2019 Nobel Prize in Physics.
As the name suggests, Neptuanian planets are planets similar in size to Neptune and Uranus. These planets also have helium or hydrogen atmospheres with cores made of rock or metal.
To date 1473 Neptunian planets have been discovered. Few Neptunian planets include HAT-P-26b and GJ 436 b. GJ 436 b is known as a “Warm Neptune” because it is much closer to its home star than Neptune is to our Sun.