Page:Illustrated Astronomy.pdf/84

 TRANSITS

The eight planets of the Solar System orbit around the Sun practically in the same plane, with a few differences between them, orbiting in the same direction. Also, out of the eight planets, six of them rotate on its axis in the same direction of its orbit (Venus and Uranus are the exceptions). All of this would make sense if the planets were formed in a protoplanetary disk 4,500 million years ago.

The Moon is “almost” aligned in this same plane, but there is a minor inclination of only 5° regarding the plane where the Earth transit the Sun (called ecliptic). It may seem not much, but it is enough so that instead of having a solar and lunar eclipse every month, we have a total solar eclipse every eighteen months on average, as we have already seen.

There are times when Venus or Mercury pass in front of the Sun, and we can see it with the right telescopes. They can’t eclipse the Sun entirely due to the considerable distance that separates the Earth from these two planets, blocking a small part of the sunlight, which are called transits.

The outer planets (from the Earth outwards), which means, Mars, Jupiter, Saturn, Uranus, and Neptune, are never between the Earth and the Sun, so they never produce a transit.

There are also other very interesting transits. Today, we know that it is very usual that stars have planets, which are known as exoplanets or extrasolar planets.

Such is the case that there should be more planets than stars in our galaxy.

The same as eclipsing binary stars, some of these exoplanetary systems may have an orbit aligned with the Earth.

In these cases, planets partially block the light coming from the orbiting star. Unlike binary stars, in which the change in the light we receive is profound, in the case of planetary transits, the variations can be one hundredth if it is a planet like Jupiter and a few millionths in the case of a planet like Earth.

Even though it is not an easy thing to perceive, since we are talking about detecting a firefly next to a stadium spotlight located hundreds of kilometers away, we can do it thanks to telescopes and cameras specially designed to measure these changes in brightness. Actually, most of the planets have been found by this technique, either from telescopes on Earth or space ones.

The Kepler space telescope, which is currently on the TESS mission and, in the near future the PLATO mission, have been and will be responsible for discovering new planets, such as the Earth, orbiting other stars, and bringing humanity one step closer to the search for another possible Earth where life may exist because, despite the progress achieved, the most crucial question remains unanswered: Is there life beyond our planet? 24 • • •