40 Exoplanets-High

Exoplanets FOCUS Question How do we find planets outside of our solar system?

What Is an Exoplanet? People who lived long ago could look up at night and see Mercury, Venus, Mars, Jupiter, and Saturn— planets of our solar system. After telescopes were invented, astronomers discovered Uranus, Neptune, dwarf planets like Pluto, and many moons. Then people wondered: Are there more planets beyond our solar system? We now know that the answer is yes! A planet is a large object, made of rock or gas, that orbits a star. An exoplanet is any planet outside our solar system. Exoplanets do not orbit the Sun.

Astronomers look for exoplanets because they want to know what the universe is made of and how it works. There is another reason people are interested in exoplanets. They want to know: Are we alone? It would be amazing to discover planets in other solar systems with kinds of life different from those that exist on Earth. Astronomers have discovered thousands of exoplanets. Some of them are small and rocky, like Mercury, Venus, Earth, and Mars. Others are large and made of gas, like Jupiter, Saturn, Neptune, and Uranus. Some seem to have watery atmospheres like Earth's. Others appear black, with atmospheres full of dark particles that are rich in carbon.

How We Find Exoplanets There are billions of stars in the Milky Way galaxy, and billions of galaxies in the universe. With so many stars in the universe, it seemed certain that there must be other planets out there. Until the 1990s, however, there was no evidence. Exoplanets are hard to see. They are relatively small and they are very far away. Planets do not give off their own light. In order to see them, we must see starlight that they reflect. If we look toward a star to see a planet, the light from the star usually hides it. Scientists have come up with ways to search for exoplanets. One way is known as the transit method. As astronomers observe a star, it dims slightly when a planet passes between the star and the telescope. The planet blocks some of the starlight. Astronomers look for stars that dim and brighten in a regular, repeating pattern. This tells them that a planet is orbiting the star.

As a planet orbits a star, the gravitational pull of the planet causes the star to move as well. In the astrometric method, astronomers measure the exact position of the star in space and how it wobbles in a repeating pattern over time. The Doppler method observes this wobble by recording the color of light from the star. Stars moving toward Earth appear bluer, and those moving away appear redder. A star with a nearby planet may appear bluer as it wobbles toward us and redder as it wobbles away. It is sometimes possible to observe an exoplanet directly. Astronomers block the light from the planet's star so that they can see objects around it. They can also observe planets directly by looking through more than one telescope at the same time, and combining information from the telescopes.

Scientists are not satisfied with simply detecting planets, however. For each planet they discover, they want to know: How big is it? How far from its star is it? What is it made of? Does it have an atmosphere? What is the surface like? Knowing these things about planets increases our understanding of the universe. To answer these questions, scientists observe different forms of light coming from the planets, using many different ground-based telescopes and space telescopes. They watch carefully to see how the planets move in space. Astronomers used the Keck and Gemini telescopes in Hawaii to find four giant, red planets that orbit a star called HR 8799. The planets were found by blocking the starlight coming from HR 8799. Observers found planet HD 189733b when it made a transit in front of its star. Astronomers used the Spitzer Space Telescope to study the infrared light coming from the planet. They discovered that there is water vapor in its atmosphere.