What is an Exoplanet?

30 years ago we knew for certain of 8 planets in the Solar System. On the 9th January 1992, the first confirmed planet orbiting another star was announced. A planet orbiting another star is called an exoplanet. Their discovery changed the way we think about the universe. Overnight it was proven that our solar system was not the only one in the universe.

Artist impression Image: Nasa/JPL

It had been long thought that there would be exoplanets elsewhere in the universe. Most scientists felt it was unlikely that our sun was the only star to have planets but for long time we lacked the technology to find them. Now we know that rather than being the exception, most stars have planets. The latest confirmed number of planets was just over 4,900.

The vast majority of exoplanet have been found using a technique called the transit method. This method involves watching the star for any sign of a change in brightness. This is fairly easy to do and once it is done accurately, it is possible to spot the small dip in brightness when a planet passes between Earth and the star. This method has been utlilised by many survey telescopes and satellites such as Kepler and has led to a huge amount of exoplanet discoveries. 2,700 of the 4,900 exoplanets confirmed were discovered by Kepler alone. One of the disadvantages of this method of discovery is that unless the planet passes directly between the star and Earth we wont see the dip in brightness.

This not the only method to discover exoplanets. Another method of finding these planets is called the radial velocity method. The star is monitored very closely for a slight wobble. This is caused by the planets orbiting the star pulling very slightly on the star. The wobble causes the light to change colour slightly which can be detected on Earth (Doppler effect). The advantage of this method is that it doesn’t matter if the star doesn’t pass in front of the star. However, very accurate observations of the stars are required and there are limits to the size of the planets that can be detected. In the early days of exoplanet hunting the radial velocity method was the most successful. The vast majority of exoplanets have been discovered using these methods.

Example of a plot of radial velocity that astronomers use to find planets. image: NASA/ JPL

In some cases it has been possible to discover exoplanets by directly imaging the planets in place around their parent star. This is far from easy. The star is much brighter in comparison to the star. It’s like trying to look at a birthday cake candle in front of a searchlight. The technology for doing this is still in its infancy. Usually a camera is designed with a type of shade called a coronagraph. This blocks out the light from the star so the planet can be seen. Kind of like holding your hand up to block the light from the searchlight. This method could be very important in the future as it has the advantage of being able to find information about the planets atmosphere.

Gravitational microlensing is another method of exoplanet discovery but is difficult to understand. When Einstein came up with his theory of gravity, he predicted that gravity would bend light around large objects. This was proven to be correct. Sometimes, an exoplanet can bend light from the star and this makes the star look brighter for a short period of time. The final technique is called astrometry. Astrometry is where the positions of the stars are very accurately recorded. When these measurements are repeated, it is possible to see the tiny movement of the star caused by the pull of the planet.

Artist concept of 55 Cancri e

The discovery of exoplanets has allowed scientists to learn lots about solar systems, including our own. Before the discovery of planets around other stars the only system we had to learn from was our own. By looking at other solar systems in various stages of their evolution, scientists are able to learn about how they form. They have also been able to look at the type of stars that have planets and it turns out that nearly any type of star can have planets. We also now know that our solar system is actually not typical and other solar systems tend to have planets with more even sizing and spacing.

Some exoplanets that have been discovered are extremely strange worlds and nothing like we experience here on Earth. One example is called Kepler 16B, this planet is strange in that it orbits two stars. That means if you looked up during the day there would be two suns in the sky. Even thought it has two suns, the planet is pretty cold and is made of gas. Another example is called 55 Cancri e. It has a surface of Lava and so is completely uninhabitable. A final example is Kepler-22b, which is a “Super Earth”. This planet is about 2.4 times the size of Earth and is positioned at the correct distance for liquid water to be present. Astronomers think there could be large oceans on its surface.

This is just a taste of the strange and wonderful places that are Exoplanets. Who knows what we might discovery in the years to come as telescopes get larger and the James Webb space telescope come online this year?!

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