The ones we have found so far fall in four broad categories. Especially with the new James Webb Space Telescope now operational, we can expect many more exoplanet confirmations. Even that number will almost certainly increase. But in just over 30 years, we have confirmed the existence of 5,000+ exoplanets. Or put it this way: until 1992, we knew our universe to contain only the eight planets that sail around our Sun, forming our solar system. That’s because even though astronomers have long suspected the existence of exoplanets, and the sheer number of stars out there suggests such existence as well, the first confirmed exoplanet was discovered only as recently as 1992. Of those, nearly 4,000 were found using transit photometry. Or the planet might be so small in comparison to the star, or so far from it, that its transit doesn’t lower the star’s brightness quite enough for us to detect, here on Earth.
For example, if the plane of a planet’s orbit around the star is perpendicular to our line of sight, we will never see it transit the star. Admittedly, there are possible planets which this method will miss. So, the search for exoplanets using this transit method is a search for stars whose brightness fluctuates regularly. She has probably detected the existence of our own planet, Earth. To understand this, imagine an astronomer in some other corner of the universe who trains her telescope on our Sun and finds that its light dims once a year. And what if the star dims and returns to its usual brightness regularly? Well, that object might be a planet, orbiting the star, transiting across it regularly. Analogously, any given star we choose to observe has a particular brightness-but what if that brightness dims? One explanation for that is that an object between us and the star has appeared to travel across-“transit"-the face of the star. As it does, it blocks some of the sun’s light-sometimes all-from reaching us. It happens because to us on Earth, the moon appears to travel across the face of the Sun. There are different techniques, but perhaps the most common-meaning the method that has found us the most exoplanets so far-is known as “transit photometry". Before I tell you what the number is today, spare a thought for how we find exoplanets.
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