There are Planets Out There!

kepler telescope planet discoveries
This artist's concept depicts select planetary discoveries made to date by NASA's Kepler space telescope. NASA/W. Stenzel

Worlds "Out There"

It wasn't all that long ago that the idea of extrasolar planets — distant worlds around other stars — was still a theoretical possibility. That changed in 1992, when astronomers found the first alien world beyond the Sun. Since then, thousands more have been found using the Kepler Space Telescope. Until mid-2016, the number of planet candidate discoveries stood at nearly 5,000 objects thought to be planets. Once a planet candidate is found, astronomers do further observations with other orbiting telescopes and ground-based observatories to confirm that these "things" are indeed planets.

What Are Those Worlds Like?

The ultimate goal of planet hunting is to find worlds like Earth. In doing so, astronomers might also find worlds with life on them. What kinds of worlds are we talking about? Astronomers call them Earth-similar or Earth-like, mostly because they are made of rocky materials as Earth is. If they orbit in their star's "habitable zone", then that makes them better candidates for life. There are only a few dozen planets that meet all these criteria, and could be considered as similar to be habitable and Earth-like. That number WILL change as more planets are studied. 

So far, less than a thousand of the known worlds could be similar to Earth in some way. However, none are Earth's twins. Some are larger than our planet, but made of rocky materials (as Earth is). These are usually referred to as "super-Earths". If the worlds aren't rocky, but are gaseous, they're often referred to "hot Jupiters" (if they're hot and gaseous), "super-Neptunes" if they are cold and gaseous and larger than Neptune. 

How Many Planets in the Milky Way?

So far, the planets that Kepler and others have found exist in a small part of the Milky Way Galaxy. If we could turn our telescope gaze to the entire galaxy, we'd find many, many more planets "out there". How many? If you extrapolate from the known worlds and make some assumptions about how many stars can actually host planets (and it turns out many can), then you get some interesting numbers. First, on average, the Milky Way has about one planet for each star. That gives us anywhere from 100 to 400 billion possible worlds in the Milky Way. That includes all the types of planets.

If you narrow the assumptions a bit to look for worlds were life might exist — where the worlds exist in their star's Goldilocks Zone (temperatures just right, water can flow, life can be supported) — then there could be as many as 8.5 billion planets in our Milky Way. If they all exist, that's a huge number of worlds where life could exist, peering out at the sky and wondering if there are other beings "out there". We have no way of knowing how many alien civilizations there are until we find them.

Now, of course, we haven't found any worlds with life on them yet. So far, Earth is the only place we know of where life does exist. Astronomers are searching for life on other places in our solar system right now. What they learn about that life (if it exists) will help them understand the chances for life elsewhere in the Milky Way. And, perhaps, in the galaxies beyond. 

How Astronomers Find Other Worlds

There are several methods astronomers use to search out distant planets. The one Kepler uses watches for flickering in the brightness of stars that might have planets around them. The decreases in brightness happen when planets pass in front of, or transit, their stars.

Another way to search out planets is to look for the effect they have on starlight from their primary stars. As a planet orbits its star, it induces a tiny wobble in the star's own motion through space. That wobble shows up in the spectrum of a star; determining that information takes painstaking study of the wavelengths of light from the star. 

Planets are small and dim, while their stars are large and bright (by comparison). So, just simply looking through a telescope and finding a planet is very difficult. Hubble Space Telescope has spotted a few planets this way. 

Since the discovery of the first planets outside our solar system more than two decades ago, researchers have resorted to a laborious, one-by-one process of verifying suspected planets. It means that astronomers have to observe, observe, and do more observing to learn more about the orbit of the possible planet, plus any other characteristics it may have. They can also apply statistical methods to large numbers of planet discoveries, which helps them understand just what they've found.

Of all the planet candidates found to date, nearly  3,000 have been verified AS planets. There are many MORE "possibles" to be studied, and Kepler and other observatories continue to search out more of them in our galaxy.