A solar eclipse happens when the Moon passes exactly between the Sun and Earth, casting a shadow on part of Earth’s surface. The Sun then appears partially or totally blacked out from our point of view on Earth.
A similar phenomenon takes place when a planet passes in front of its star: the passage is called a transit. From Earth, we can see Mercury and Venus transit our star, the Sun. We can also detect and study exoplanets, which are planets beyond our solar system, when they pass in front of the stars they orbit. Obviously, these exoplanets do not cast a shadow on our planet, but their passage blocks out some of the light of their star.
Using instruments like those aboard the James Webb Space Telescope, scientists can observe these transits to discover intriguing details about alien worlds – including whether they are likely to harbour life.
An exoplanet’s fingerprint
Most of the thousands of known exoplanets have been discovered not through direct observation, but by analyzing the brightness of their host star. Using the transit method of detection, scientists hunt for undiscovered exoplanets by detecting periodical dips in the star’s light, which may be a sign that an exoplanet is transiting between the star and our vantage point.
But the transits may reveal more than just the presence of alien worlds. They also help scientists determine whether life could exist there.
An exoplanet’s solid core will completely block out starlight. However, some exoplanets have thin atmospheres, just like Earth. These atmospheres let most of the starlight pass through, but certain parts will be absorbed by elements like oxygen, water vapour or carbon dioxide. These absorbed parts of the starlight are like a unique “fingerprint” of each exoplanet’s atmosphere. Astronomers are able to determine if the exoplanet may be habitable, or a candidate for the possible presence of life.
