The universe is an amazingly large place. It's so large that we may sometimes feel as though we are lost in a vast, empty sea. But of course, the cosmos is anything but empty. The universe is populated with asteroids, planets, stars, black holes, and a plethora of other objects. This seems rather obvious; however, we didn't always know this. In fact, we didn't even discover Pluto until 1930. Then, for more than half a century, the universe was silent.
The first exoplanet (a planet that orbits a star in another solar system) wasn't detected until the early '90s. At this point, Dr. Alexander Wolszczan, a radio astronomer at Pennsylvania State University, reported what he called "unambiguous proof" of extrasolar planetary systems (of course, scientists believed that other planets existed before this, but believe and proof are terribly different). Notably, the first detection, the one made by Wolszczan, involved planets orbiting a pulsar. Due to the extreme radiation given off by these stellar objects, there was no chance of life existing on the first exoplanets that we discovered.
Very shortly thereafter, a Swiss team led by Michel Mayor and Didier Queloz of Geneva announced the discovery of the first alien world orbiting a star like our own sun. Mayor and Queloz's work gave us hope—at last, we had found alien worlds that lived in a system that was at least somewhat akin to our own, making the likeness of alien life much more plausible.
Then there was a torrent of planets. To date, we have discovered more than 1,000 alien worlds, with another 3,200 planetary candidates, 90 percent of which should end up being confirmed, according to mission scientists.
This animation reveals just how empty our universe seemed only a few decades ago, and how populated it is now. The animation was made by Hugh Osborn, who is a PhD student at the University of Warwick.
On his website, Osborn asserts, “The idea of this plot is to compare our own Solar System (with planets plotted in dark blue) against the newly-discovered extrasolar worlds. Think of this plot as a projection of all 1873 worlds onto our own solar system, with the Sun (and all other stars) at the far left. As you move out to the right, the orbital period of the planets increases, and correspondingly (thanks to Kepler’s Third Law), so does the distance from the star. Moving upwards means the mass of the worlds increase, from Moon-sized at the base to 10,000 times that of Earth at the top (30 Jupiter Masses).”
So. How do we detect planets that are, potentially, thousands of light-years away? There are a few different ways. One way is the radial velocity method.
As is the case with the Sun and the other planets in our solar system, other stars with orbiting planets behave strangely in response to the planet’s gravity. Subsequently, the slight gravitational perturbations not only cause the star’s orbit to shift, but they lead to variations in the speed we see the star moving in when it travels towards or away from Earth. This, in turn, results in a displacement in the parent star’s spectral lines; otherwise known as the Doppler effect.
If we are trying to uncover stars that are farther away, we generally rely on the transit method. Unfortunately, this method doesn't give us information about the planets mass, but it does give us information about the radius of any orbiting planets.
This works by observing the planet as it passes in front of its parent stars' disk from our vantage point. When this happens, the planet obstructs a small amount of the star's light. The amount of light that dims as this transit occurs is dependent upon the relative sizes of the star and any orbiting planet. Planetary transits are only observable for planets whose orbits happen to be perfectly aligned from our vantage point...however, it seemingly works best for scanning large areas of the sky with a multitude of stars at once.
In recent years, astronomers have concocted several new ways we can suss out other planets. The methods include stellar fingerprinting.
This method looks at the chemical composition of stars to see if it contains a decent amount of iron and other heavier elements. Current models (along with observation) tell us that most heavy elements and compounds are locked up in rocky bodies following the formation of planetary systems. Even more-so when protoplanets are involved. They are born once they accrete enough heavy material for a planet to coalesce; meaning that the material doesn't make its way into stars, leaving them anemic (so to speak). Thus, scientists can merely view the chemical composition of the star, looking to see just how much heavy elements individual stars have. The more, the less likely a planet was born. Less and the likelihood is greater, which gives astronomers a starting point.