In 1991, exoplanets were merely a theory. It was assumed that since our star had planets so would other stars. But no one was sure. The closest star to us is 4.2 light years away, so it’s hard for any telescope to see a planet that doesn’t emit light that far away.

Then in 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced that they had discovered two planets orbiting a star called pulsar PSR 1257+12. Follow up observations confirmed the planets exist. In 1994, a third planet was found in the stars orbit.

A lot of the earlier exoplanets were detected by measuring gravitational influence. Just as the star pulls on the planets, the planets exert a tiny pull on the star. This was a good way of finding very large exoplanets but not much else was known other than something large was pulling on the star.


An example of Spectroscopy as white light passes through a prism and separates the light into it’s component colors.

As telescopes such as Hubble and Kepler became available, spectroscopy became more widely used and refined. If you have ever played around with a prism to make rainbows, then you can understand how it works. Every element produces a different signature as light is absorbed or emitted from it. Knowing this we can split the light from other stars into the spectrum, allowing us to know what the star is made of. The same thing goes for exoplanets. When more powerful telescopes came online, scientists where able to see planets transit their host star. We can tell what a planets atmosphere consists of by the light of its star shining through its atmosphere.

What are Exoplanets?

Was I getting ahead of myself? What exactly are exoplanets? Exoplanets are any planet not orbiting our sun. They are typically found orbiting other stars but many exoplanets are simply drifting through space without a star. These planets aren’t as fascinating as life would unlikely exist there. Honestly, one of the major reasons for studying exoplanets in the first place is to look for life or the conditions for it.

How many exoplanets are out there?

To date there are over 5000 exoplanets confirmed. It is assumed that there are many more planets than there are stars in the universe. That’s a lot of planets! They come in many forms. Some are so large that they have nearly become stars themselves, called brown dwarfs. Brown dwarfs emit a small amount of light and are not much bigger than Jupiter, but they are much heavier.

Size matters

Size and density are the only differences between a star and planet. Once a planet becomes large enough they get so dense that nuclear fusion triggers in their core. Then voila, a star is born. Jupiter is often referred to as a failed star for this reason. But it would need to be around 9 times larger to trigger fusion. So it’s a bit of a misnomer but still a really cool thought experiment. It’s hard to imagine a planet becoming a star. But that’s how it happens.

4 Types of exoplanets

Exoplanets have been categorized into 4 types: Gas giants, Neptunian, Super earth, & terrestrial.

Gas giants are similar to our own Jupiter or Saturn in size and density. Neptunian planets are similar to Neptune in terms of size and they are comprised of mostly hydrogen and helium. Super earths are rocky worlds much larger than earth. Terrestrial planets are the holy grail, being similar in size to earth and may or may not have atmospheres.

We wouldn’t be able to live on the first 3 types of exoplanets. We could perhaps live on one of the moons of the larger planets. The terrestrial type planets hold the most promise for habitability. Unfortunately, they tend to be the hardest to detect. Finding a terrestrial or even a super earth within its stars habitable zone is where we will find the best prospects of another earthlike planet.

The closest prospect found to date is Kepler-452b. Also referred to as Earth 2.0. It is found orbiting the star Kepler-452 some 1800 light years from Earth.

Discovery Methods

There are currently 5 ways astronomers can detect exoplanets.

  • Radial Velocity – Watching the host star for a gravitational wobble.
  • Transit – Watching for the planets shadow to pass in front of the star. (Most Prominent)
  • Direct Imaging – Directly viewing the planet.
  • Gravitational Microlensing – The host stars light bending around the planet.
  • Astrometry – Small movements in the stars wobble.


Exoplanet Missions

The Kepler space telescope found more than half of all discovered exoplanets to date. It has found even more potential candidates that have yet to be confirmed. Hubble was the first telescope to directly image an exoplanet. While the Spitzer telescope didn’t find many planets, scientist were able to use techniques learned with the telescope, and apply them to more advanced telescopes.

JWST has just came online and it is already imaging exoplanets. The overwhelming majority of planets discovered have happened in the last 5 years. So with JWST and the upcoming Nancy Grace Roman Space Telescope. I’m quite confident that the number of discovered planets will go from a few thousand to at least tens of thousands over the next decade. The next decade is going to be exciting for astronomy.

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