It was almost 100 years ago Clyde W. Tombaugh discovered Pluto. It was the last planet discovered until 1992, when humans discovered another one. But this fresh planet wasn’t in our solar system – it was orbiting another star. We call this an extrasolar planet, or an “exoplanet” for tiny.
Since then, astronomers have cataloged more than 6,000 exoplanets. If you think it’s arduous to remember the names of our own planets, give this a try All planets with names like HD 189733b. (A cheerful place where molten glass rains and the wind blows at 9,000 kilometers per hour.)
Even the closest exoplanets are more than 4 light-years (36 trillion miles) away, making it doubtful that we will ever visit one. So why bother? This is because it helps us answer the age-old question: are we alone in the universe? As far as we understand, life requires a planet and the race is on to find one with Earth-like characteristics.
Why are they demanding to find?
The problem is that you can’t just grab the best telescope and start looking around the sky. Telescopes have a confined resolving power – the smallest angular size they can “see”. In the case of the Hubble Space Telescope, this is 0.05 arcseconds, which is incredibly tiny – about 1/72,000 of a degree. The HST was able to spot a giant Jupiter-sized planet 590 billion kilometers away. Amazingly, it’s only 0.06 light-years away and the nearest star, Proxima Centauri, is 4.25 light-years away.
Another problem is the darkness of the planets. Sure, Jupiter is simple to see in our night sky because sunlight reflects off its surface. However, during the day, Jupiter cannot be seen at all because the reflected lithe is much weaker than direct sunlight. The same applies to exoplanets. When we look at the lithe from a star, the planets around it are not radiant enough to be seen.
Fortunately, there are other methods. Below I will explain the two that were used to find most of the exoplanets we know today. There’s a lot of frosty physics here, so let’s go!
Orbits, wobbling stars and celestial shifts
What happens when a planet moves around a star? First, there is a gravitational pull that pulls the planet toward the star. The magnitude of this force (FG) depends on the mass of the star (M) and planet (M), as well as the distance (R) between them:
Illustration: Rhett Allain