고등학교 영어Ⅰ (미래엔 김성연) 연계 글

For centuries, people wondered whether other stars had planets, but such planets are extremely hard to find. A planet produces no light, and the faint starlight it reflects is lost in its star's glare. Astronomers therefore search for planets indirectly, by studying the small changes that a planet causes in its star's light.

A planet's gravity tugs on its star, so the star wobbles slightly, approaching us and then moving away. It is well known that light from an object moving toward us shifts toward blue, while light from an object moving away shifts toward red. Even Jupiter, the largest planet circling our sun, moves the sun at only about 12 meters per second. This approach, called the radial velocity method, therefore demands extremely sensitive instruments.

In October 1995, the Swiss astronomers Michel Mayor and Didier Queloz announced that they had used this method to find the first planet orbiting a sunlike star. The planet, 51 Pegasi b, is a giant ball of gas like Jupiter, yet it circles its star in only four days. The finding came as a shock, because it had been widely believed that such large planets could form only far from their stars. Soon afterward, two American astronomers confirmed the result with their own telescope.

A second approach, the transit method, works only when a planet's orbit happens to carry it between its star and Earth. Whenever the planet passes in front of the star, a tiny fraction of the starlight is blocked, and the star appears slightly dimmer until the planet moves on. The amount of dimming shows the planet's size, while the radial velocity method can reveal its mass. Once both values have been measured, astronomers can calculate the planet's density and learn what kind of world it is. Starlight that passes through a planet's atmosphere during a transit can also be studied for clues about that atmosphere.

Since 1995, thousands of planets beyond our solar system have been confirmed, many of them found by the transit method. Many of these worlds look nothing like the planets that circle the sun, and as a result, ideas about how planets form have been revised. In 2019, Mayor and Queloz were honored for their discovery with a share of that year's Nobel Prize in Physics. Today, it is thought that most of the several hundred billion stars in our galaxy have planets, so many more worlds may still be waiting to be found.