Weighing a planet is a tough task. It's not like you can just put them on a bathroom scale. And, while astronomers figured out how to measure the mass of planets in our solar system a long time ago, it's practically impossible to weigh exoplanets. Well, it was until recently.
MIT planetary scientists Julien de Wit and Sara Seager just devised a method that enables them to weigh an exoplanet by measuring the starlight that shines through its atmosphere. When an exoplanet passes in front of its star, it causes a blip in the amount of light that shines toward Earth. This is actually how we're able to discover exoplanets in the first place.
Not all of the starlight zooms past the exoplanet, though. Some is actually filtered through the atmosphere, and by measuring the spectrum of that light, the MIT scientists are able to learn all kinds of things about the planet, such as atmospheric pressure, temperature, and gravitational pull. With that information, they've come up with a new method for calculating not just atmospheric chemistry but a planet's weight and mass.
So why does this matter? Well, it's always been really difficult to learn details about exoplanets. They're really far away! With this tried and true starlight method, though, astronomers can now also deduce whether the planet is gassy like Venus or rocky like Earth. It also makes it easier to discover new exoplanets which—let's cut to the chase—brings us that much closer to finding an Earth twin.