Exoplanets, also called as “extrasolar planets” are the planets that exist outside our solar system that orbit stars other than our sun. They might be in the Milky Way or in other galaxies. As of 2018, more than 3500 exoplanets have been confirmed discovered from ground-based and space-based observatories. Astronomers estimate that there could be trillions of planets around other stars.
Some famous Exoplanets
- First ever exoplanet was discovered in 1992. These were two planets orbiting the Pulsar – PSR 1257+12.
- ‘PSR B1620-26b’ is the oldest exoplanet known in existence. It is as old as 13 billion years and was first discovered in 1993.
- The closest exoplanet to the earth is Alpha Centauri Bb and is just 4 light years away.
- Kepler-78b planet is just at a distance of 1 million miles from its star which is the shortest known distance between any star and its planet. Its orbit is so small that it completes the entire revolution around the star in just 8.5 hours.
- The Dark Planet: TrES-2b does not reflect any light and hence been named as “The Dark Planet”. It reflects a meagre 1% of the light projected on it.
- Janssen is the densest planet discovered to date.
- Planet revolving around a sun-like star: 51 Pegasi b is the only planet known to be revolving around a star that is similar to the Sun.
- The hottest planet known: TrES-4 is the hottest planet known to the astronomers. The temperature on its surface is about 2,300° F (1,260° C). It is so hot because it is so close to its star. It also the largest exoplanet known so far. It is about 1.7 times the size of Jupiter. It is so close to its star that it completes its orbit in 3 days!
- The planet that takes 900,000 years for 1 revolution: 2MASS J2126-8140, which is 82 light years away from us and 620 million miles away from its star takes 900,000 years to complete one revolution.
- The planet with burning ice, Gliese 436b has a surface temperature of about 570°F(300°C). What keeps the water in its solid form is extreme pressure conditions in its environment.
- The Planet orbiting two stars PSR B1620-26b, also called “Methuselah” is 12,400 light years away from the earth. It orbits two binary stars in a circumbinary orbit.
- The Diamond planet, 55 Cancri e is largely made of Carbon and has density twice of the earth. The temperature and pressure on the planet is so high that astronomers estimate it must be full of diamonds. This planet is also very close to its star and takes about 18 hours to complete its orbit.
- The planet with glass storms, HD 189773b is a pretty blue colored planet just like our earth. However, the extreme temperature(1800°F/1000°C) and pressure transform the silicate in the atmosphere into glass that whooshes around with wind gushing at the speed of 4000 miles per hour.
How do astronomers find exoplanets
Radial Velocity Method:
The method uses the Doppler Effect to detect the exoplanets. It first detects the stars by observing how the color of the spectral light received from the star varies as it moves depending on whether the star is moving away (red) from us or towards (blue) us.
This method is quite effective for detecting planets orbiting small or medium-sized stars since the velocity of these stars is slowed down by the gravitational force of their planets, hence giving clearer spectral lines.
Recently in 2016, TRAPPIST 1 was discovered using this method.
Transit Photometry Method:
This method uses variation in the amount of light received from the planet. The variation is usually caused by a planet obstructing the light to reach us. It is a very simple method and also highly effective.
This method is used for detecting planets that are 100s of light years away from the earth. The method is based on the Microlensing effect that causes the light from a star to bend while its path is crossing near another star or planet-star system. Depending on the size of the planet and its star, the changes in the light curve vary and help us identify a potential planet or a star.
3 Types of Exoplanets
To understand planets better, astronomers have classified exoplanets into 3 types – rocky planets, gas dwarfs and, ice giants. Out of these only rocky planets can be habitable.
Rocky exoplanets as the name explains have a firm rocky and a solid surface implying that the temperatures are somewhat similar to earth. Some of the known rocky exoplanets are:
The planet is habitable temperature-wise and is also almost similar to earth in terms of size.
This is another Earth-like planet and can be habitable.
55 Cancri e:
It is a rocky planet. Astronomers estimate that it might have water though in a steamy form because it so close to its star as already mentioned above. This planet orbits its star in 18 hours.
Gas Dwarf Exoplanets
These planets have a rocky core, however, their rocky core is enveloped by a thick layer of gases like Hydrogen and Helium.
Gas Giant Exoplanets
A good example to understand what these planets are like would be Jupiter and Saturn. Gas giant planets are also known as failed stars because they contain the same basic elements as stars do. An example of a gas giant exoplanet would be TrES-4, which as already discussed before is the largest exoplanet known.