Space is huge.
Really, really huge.
Occasionally, here and there, you’ll stumble across an asteroid, or planet, or perhaps even a star. But all in all, space is exactly what its name states: empty space.
It’s difficult to fully appreciate just how gargantuan the universe is, and, beyond that, just how spacey it is. This interactive site gives an idea of how far apart the planets in our solar system are – which itself in its entirety can be represented as no more than a singular dot on a map of our galaxy.
All this said, a question can arise: how on Earth, Jupiter, or Pluto have astronomers been able to find exoplanets millions of light years away?
It’s a good question, but don’t hasten to underestimate the power of human creativity. Astronomers have many clever methods to aid them in their never-ending hunt for planets outside of our familiar bubble.
Stars are massive. They can easily dwarf the planets that orbit them by multiple orders of magnitude. Jupiter, our largest planet (and theoretically one of the largest possible, since any additional mass would cause gravity to crush it into a smaller sphere), could fit snugly inside our Sun a thousand times. In fact, the Sun consists of 99.8% of the entire solar system’s mass.
And yet, despite the Sun’s desperate need for a diet, the gravitational effects of its planetary companions are not completely negligible. Just as the Sun tugs on the planets, comets, and asteroids that orbit it, they too tug on it. Not nearly as much, of course, but they nonetheless do.
An easier way to think about this is by using the concept of the center of mass. An object will never truly simply “orbit” around another, more massive object. Instead, the two orbit around a common “center of mass” – a weighted average of the two masses.
In short, the Sun isn’t just sitting still. The center of mass for the solar system is located inside the Sun, but not quite perfectly in the center of the Sun. Because of this, the Sun will appear to “wobble” as it orbits around the center of mass caused by the influence of the other planets nearby.
Although planets are far too dim and far too small to be directly detected by our current technology, we can fairly easily detect these “wobbles” of stars. Through it, we can infer that there must be some objects of appreciable mass that are orbiting the star and causing it to wobble, rather than stay in one fixed location (relative to the distant stars around it).
Simply put, sometimes we do have the ability to detect a planet if it is in plain sight and not completely obscured by the light of a star nearby. Although this is incredibly rare, and generally not the way astronomers detect exoplanets, given the sheer size of the universe it is unavoidable that we will find a handful of planets in this fashion.
Just like how the Moon can pass in front of the Sun and cause an ethereal solar eclipse, a planet can pass in front of its parent star and (at least partially) block out its light.
This little “blip” in the star’s light must be indicative of an object passing by it and obscuring some of the light. Of course, great care must be taken to ensure that the dark spot was not caused by some technological malfunction or by some other phenomena unrelated to an exoplanet’s behavior. However, with our current understanding, equations, and technology, transits provide a fairly reliable tool for us to be able to detect the presence of planets very far away that cannot be seen with regular telescopes.
There are a myriad of other ways that astronomers can infer the existence of a planet, and more techniques are being developed every day. These are simply some of the most frequently-used ones, and will very likely be what we use to, one day, find a planet that hosts life like ours.