There are eight major planets orbiting our sun. We know them as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. We can see five of these planets without the aid of a telescope. For most of human history, these were the only planets we knew of in the universe.
But are there planets orbiting other stars? This was not an easy question to answer.
With better and better telescopes, maybe we could have just looked for planets orbiting around other stars. But stars are very bright, while planets are very small and dim, so the bright light of a star makes it impossible to see dim planets, even with the best telescopes.
However, there is a way that we can use the brightness of stars to our advantage. We can attach instruments to telescopes that measure the brightness of a star very precisely. If the star is being orbited by a planet that is in line with the Earth, the planet will pass in front of the star as seen from Earth. We call this event a “transit.”
When the planet crosses in front of the star, it blocks some of the light coming from that star. If we measure the light coming from the star over time, we will see a tiny dip in the star’s brightness as the planet crosses in front of it. A graph of the brightness of the star over time is called a “light curve.”
We can see a similar event when one of the inner planets, Mercury or Venus, occasionally passes between the sun and the Earth. During one of these transits, we see a small, black dot move across the sun. The planet is a small black dot because it is blocking part of the light of the sun.
Once an exoplanet (a planet outside our solar system) is detected with this method, scientists can determine the planet’s orbital size from the “period” of its orbit, which they find by looking at the light curve. Because the planet is orbiting the star, the dip in the star’s brightness will occur every time the planet crosses in front of the star. This occurs on every orbit, so the period of the orbit is the time between dips.
The size of the planet can be determined from the depth of the transit — the amount the brightness of the star drops — and from the size of the star. From the orbital size and the temperature of the star, the planet’s temperature can also be calculated. From this information, the question of whether or not the planet is habitable (which does not mean it is inhabited) can be answered.
In March of 2009, NASA launched the Kepler Space Telescope. Kepler used this transit method to find exoplanets, with a particular focus on finding planets that might resemble Earth.
During Kepler’s nine-year mission, astronomers discovered thousands of exoplanets. Most of them are between the sizes of Earth and Neptune. Kepler pointed at a small region of the constellation Cygnus for the first four years of the mission, and this is where many of the planets were discovered.
The Kepler Telescope found 2,682 confirmed exoplanets with more than 2,900 candidate planets awaiting confirmation. Many of these were “super-earths,” planets that are slightly larger than Earth but have a rocky surface. Kepler’s findings also helped astronomers to begin to group exoplanets into types, which will help us understand their origins.
The data collected by the Kepler Space Telescope have revealed that there are a large variety of planetary systems. These systems can be very compact — some would fit within the orbit of Mercury. They can orbit around two stars, much like Tatooine in the Star Wars universe. Another exciting find is that small, rocky planets similar to Earth are more common than large gas giants like Jupiter.
Kepler’s work of collecting data is finished, but another spacecraft is in position to take its place. The new exoplanet-hunting spacecraft, the Transiting Exoplanet Survey Satellite (TESS), was launched on April 18, 2018, and made its first observations on July 25. The TESS spacecraft is scheduled to scan about 85 percent of the sky during its initial two-year mission.
The answer to the question of whether there are planets orbiting other stars is Yes. The next question is, How many can we find?