Exoplanet Discovery

In 1995, astronomers made a discovery that changed everything we thought we knew about the universe. They found a planet orbiting a star that is not our Sun — a world called 51 Pegasi b, circling a star 50 light-years away. Today, thanks to NASA's Kepler Space Telescope and its successor TESS, scientists have confirmed over 5,500 exoplanets — planets that orbit stars outside our solar system. Some are scorching gas giants bigger than Jupiter. Some are icy frozen worlds. And some might be just the right temperature for liquid water — and maybe even life.

By the end of this lesson, you will be able to: • Explain what an exoplanet is and why they are so hard to see directly • Describe two main methods scientists use to discover exoplanets • Identify what the habitable zone means for a star system • Name at least two real exoplanets and describe what makes them interesting

An exoplanet (short for extra-solar planet) is any planet that orbits a star other than our Sun. Our solar system has 8 planets orbiting the Sun. But there are hundreds of billions of other stars in our galaxy — the Milky Way — and most of them probably have their own planets. Exoplanets are incredibly hard to see directly because stars are so much brighter than the planets near them. Imagine trying to spot a firefly hovering next to a lighthouse from far away. That is roughly what it is like trying to photograph a planet next to its star. So astronomers had to get creative and invent clever indirect methods to detect planets they cannot even see.

The most successful exoplanet-hunting method is called the transit method. Here is how it works: When an exoplanet passes in front of its star (from our point of view), it blocks a tiny fraction of the star's light. The star's brightness dips very slightly — sometimes by less than 1% — each time the planet crosses in front. Scientists use extremely sensitive telescopes to measure a star's brightness very precisely over weeks and months. If the brightness dips on a regular schedule — say, every 10 days — that is a strong sign a planet is orbiting the star with a 10-day period. NASA's Kepler Space Telescope used the transit method to find more than 2,600 confirmed exoplanets during its mission from 2009 to 2018. It watched over 150,000 stars at once, looking for tiny brightness dips.

Here is something surprising: a planet does not just orbit a star — the star also moves a tiny bit because of the planet's gravity. It is like a game of tug-of-war. The planet and star both orbit a common center of gravity called the barycenter. When a planet pulls a star slightly toward us, the star's light shifts to a slightly bluer color. When it pulls the star away, the light shifts slightly redder. Scientists call this the Doppler shift or wobble method. This is how the very first confirmed exoplanet, 51 Pegasi b, was discovered in 1995 by astronomers Michel Mayor and Didier Queloz. They won the Nobel Prize in Physics in 2019 for this discovery — one of the most important in the history of astronomy.

Scientists call the area around a star where temperatures might allow liquid water on a planet's surface the habitable zone — or the Goldilocks Zone. Just like Goldilocks's porridge: not too hot, not too cold, but just right. Earth sits perfectly in our Sun's habitable zone. Venus is too close — its surface temperature reaches 465 degrees C. Mars is just beyond the outer edge — too cold for liquid water on its surface today. In 2017, NASA announced the discovery of the TRAPPIST-1 system — 7 Earth-sized planets orbiting a tiny star just 39 light-years away. Three of those planets sit in the habitable zone. Scientists are studying these worlds carefully using the James Webb Space Telescope, which launched in 2021 and can analyze the gases in distant planet atmospheres.