This week, NASA announced that the 5,000th exoplanet had been confirmed. An exoplanet is an extra-solar planet, a planet orbiting around another star, not our sun in our solar system.
The first exoplanet was discovered back in 1992. However, it was a bit of a surprise as they were orbiting a pulsar called PSR B1257+12.
It is amazing to think that before the early 90s we had never known about another planet in another star system. While theories and calculations suggested they were there, we had not found one.
A pulsar is a neutron star, which is the remnant core of a big star that has already exploded. However, this star is spinning very, very fast, has a huge magnetic field, and is emitting lots of radiation. A planet orbiting around a star like this would not be a nice place to live.
In 1995, Michel Mayor and Didier Queloz found an exoplanet orbiting around a sun-like star, 51 Pegasi. Unlike the first one, PSR B1257+12, this was a more normal star, and one that astronomers had desperately wanted to find a planet orbiting around.
This was a fundamental discovery, and in 2019, Mayor and Queloz won the Nobel prize in physics for it.
The big revolution in exoplanets came in 2009 with the launch of the Kepler Space Telescope. It was named in honour of Johaness Kepler, the German astronomer and mathematician who calculated the motions of the planets in the solar system.
The Kepler Space Telescope was built so that it could stare continuously at a patch of sky for years. Every 30 minutes, Kepler took a picture, or measurement, of hundreds of thousands of stars in this single patch of sky. By doing so, it could wait for any planets orbiting those stars to cross in front of it, what we call a transit.
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Imagine a solar eclipse where the moon blocks the sun. The sun gets dimmer as the moon moves into position, and then brighter as the moon moves away. By staring at distant stars, the telescope could wait for a planet to move across the star as viewed from Earth, and look for the dip in light.
Depending on how big the dip is and how many it does over a certain time, we can determine the size of the planet and how far away it is from its star. If we want to find Earth-like planets, the smaller ones, and the planets in the habitable zone, the area that is not too close to its star where it is very hot, or too far away that is very cold, this is important to know..
But to make sure it is real, we need to see multiple transits, at least three, but usually more. While this may not seem like a challenge, if we are looking for a planet that orbits a star like Earth, then we need to wait three years to see it three times. Jupiter, which takes 12 years to orbit the sun, we'd have to wait 36 years to see it three times.
So confirming the 5000th planet means a lot of planets, around a lot of stars, and even more measurements of them. We believe that nearly every star in our Milky Way has at least one planet on average. We already know of some stars that have five to seven planets, much like our own solar system which has eight.
If there are 300 billion stars in our Milky Way, and they all have planets, there are literally trillions of planets still waiting to be found.
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