In our solar system, we have a star which is the Sun, and planets are revolving around the Sun, including the Earth which we live in. But how are the planets that are beyond our solar system? Are they exist or only existed in theory?

At first, it is only a theory, because it seems impossible to detect the planets that are far away, which are light-years away, but at last, the astronomers successfully created methods to detect the planets that are beyond from our solar system. The planets are called Exoplanets. In the solar system, the planets orbit around the Sun, while Exoplanets or extrasolar planet are planets that orbit around other stars beyond our solar system. Simply put, Exoplanets are planets that are outside of our solar system. Astronomers predict (roughly) that our galaxy contains around one exoplanet per star. Some stars have many planets, like the Sun itself got eight planets. But some stars have none.

There are many types of Exoplanets as stated by astronomers, according to the planets’ characteristics. They can be categorized as Earth-sized, Earth-like, Super-Jupiters, gas giants, rocky worlds the size of Earth, rocky giants, Super-Earths, mini-Neptunes, and gas dwarfs. For Earth-sized and Earth-like exoplanets, they are similar in size to our planet, the Earth. While Earth-like, rocky Earth-sized and rocky giants planets have multiple characteristics in common to the Earth, including similar atmospheres and also possible liquid water on the surface. Super-Earths are defined as the Earth-type planets which are larger than Earth and contain more mass than the Earth. The mini-Neptunes usually referred as gas dwarfs, usually smaller than Uranus or Neptune. They may have up to 10 times the Earth’s mass and have very dense atmospheres.

The first possible exoplanet that was detected in 1917, which orbiting the Van Maanen’s star, but as such it was not recognized, while the first exoplanets that was detected scientifically was reported as the first planet-size masses around a dead star (PSR1257+12), which are 2000 light-years away. They are the first pulsar planets that have been found, they are PSR1267+ B and C, and one of them are in Super-Earth category. Astronomers still trying to find out when these planets were formed. Astronomers also questioned how the planets survived the supernova explosion, as they are orbiting the dead star. In 1995, there comes news of the first known exoplanet, known as 51 Pegasi b. It is categorized as gas giant exoplanet, which is Jupiter-like planet, and it is orbiting its star (G-type star, a Sun-like star) closer than how Mercury orbiting the Sun, it takes 4.2 days to complete a circle orbiting its star. It is also known as “hot Jupiter” because the astronomers found out that it is a very warm gas-giant planet.

Credit: NASA

There are several methods used for detecting those exoplanets, but the most successful one is by radial velocity, also known as “Doppler wobble”, to look for the “wobbly” stars. A star that has planets does not circle around its center perfectly. This off-center orbit makes the star appear like it is wobbling from far out. However, only big planets can be seen in this way, while small planets are hard to be detected in this way, because they create only small wobbles. Besides that, there are missions launched that detected those exoplanets. NASA launched Kepler in 2009, a spacecraft to look for exoplanets. How does the Kepler detect the exoplanets?

Kepler detected those exoplanets using the transit method, another famous successful method. Transit means when a planet passes in front of its star. The star’s light will be blocked a little bit when the planet transits in front of the star. This means that a star will appear less bright when the planet passes in front of the planet. This way, astronomers will be able to observe the changes of the brightness of the star during a transit. Thus, it can help the astronomers in figuring out the size of the planet. Besides, astronomers can find out the distance between the planet and the star by studying the time between transits. For instance, Kepler mission detected a sun-like star (yellow dwarf star) named Kepler-11, located approximately 2000 light years from our Earth, with six planets orbiting around it named Kepler-11b, Kepler-11c, Kepler-11d, Kepler-11e, Kepler-11f and Kepler-11g. The planets are exoplanets as they are beyond our solar system.

An artist’s representation of Kepler-11, a small, cool star around which six planets orbit. Credit: NASA/Tim Pyle

There are thousands of planets that have been found by Kepler mission. After nine years in the space, collecting data about the planets and stars, finally the mission ended as the Kepler space telescope ran out of fuel, so in 2018, the mission ends. But there will be more planets to be found by NASA’s TESS mission (Transiting Exoplanet Survey Satellite), launched on 18th April 2018, which the purpose is to survey about 85% of the sky, which the area are 400 times larger than the area covered by Kepler, to find out the planets that are near to stars. Latest update at 28th February 2020, there are 4126 exoplanets confirmed by NASA, and 5013 planets are still candidates, which they may be the planets but still unconfirmed.


Written by: Siti Sarah Harizan
Internship Student USIM, 2020