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Twenty-four years ago, astronomers Michael Mayor and Didier Queloz started something big. They made science's first discovery of a planet orbiting a typical star outside our solar system. Their search method and their discovery launched a revolution in astronomy that has now found over 4,000 planets orbiting stars in our Milky Way galaxy. Because they opened up the field of exoplanet astronomy, Mayor and Queloz received half of the 2019 Nobel Prize in physics.

The duo searched for exoplanets by watching the starlight from 142 sunlike stars. If a planet orbits a star, physics tells us the star must also execute a small circular motion -- both the planet and the star move in circles (one big, one much smaller) around a common center. Think of the planet's motion "shaking" the star around in a small circle. This motion slightly alters the frequency -- the color -- of the star as seen from Earth. When the star, in its circular motion, moves away from Earth, the starlight looks slightly redder; when the star moves toward Earth, the starlight looks slightly more violet. If you have noticed the pitch of an ambulance's siren changing from higher to lower as it passes you, you have experienced this "Doppler shift" -- but in sound waves instead of light waves.

The duo found one star, "51 Pegasi," whose Doppler shift was sufficiently pronounced that they could detect it. The key to the discovery was a sensitive instrument that could detect the slight changes in the star's "spectrum" -- the precise frequencies of light -- emitted by the star.

To shake the star enough to cause a strong Doppler effect, the force of gravity between the star and the planet had to be large, implying the planet was quite massive ("heavy") and orbiting close to the star. The astronomers calculated that the planet, "51 Pegasi b," was a gaseous ball comparable to our solar system's gas giant Jupiter. It's much closer to its star than Mercury is to our sun. It's difficult to reconcile this planet's small orbit with astronomers' standard theories of planet formation around stars. After all, our solar system's Jupiter is the fifth planet out, and quite far from the sun. Today, many other "hot Jupiters" (giant planets orbiting near their star) have been discovered, and many astronomers now conclude that such planets originally formed in much larger (more distant from their star) orbits, and then migrated inward due to processes in their planetary system.

Nearly 700 multiplanetary systems have been discovered, most with only two confirmed planets but many with three or more planets. Besides our own solar system, there is one other known system with eight planets.

Is there life out there? Currently, the best candidate is planet K2-18b, discovered in 2015. Observations suggest it might be the first exoplanet to exhibit liquid water. This is important because liquid water is crucial to life on Earth.

It's not easy to demonstrate the existence of water on an exoplanet. The team that accomplished this probed the chemistry of K2-18b's atmosphere by observing the planet as it traversed directly in front of its star. Starlight passing through the planet's atmosphere then bears the signatures, within its "spectrum" (the frequencies and wavelengths present in the light), of specific chemicals such as water. The telescope then receives this light.

As ice and steam, water is ubiquitous in the universe, but it's not easy to make liquid water because the pressure and temperature must be just right. Although many exoplanets are known to lie in the "water zone" around their central star -- the region within which liquid water would exist on an Earth-like planet -- K2-18b may turn out to be the first exoplanet known to actually have water.

A red dwarf star, rather than a star, lies at the center of K2-18b's orbit. Smaller and cooler than our sun, red dwarfs comprise some 75 percent of the stars in our galaxy (sunlike stars comprise only 10 percent). At 110 light years away (the distance light travels in 110 years), the star is "in our neighborhood" within our galaxy. For water to exist, the planet must orbit quite near the red dwarf. The planet is eight times more massive than Earth. Any life on this planet would be quite different from life on Earth.

My guess is life exists on many exoplanets, and such life will be discovered within 25 years. For a good book about exoplanets, I suggest Elizabeth Tasker's "The Planet Factory."

Commentary on 11/05/2019

Print Headline: Scientists discover exoplanets

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