Are we alone in the Universe? For years, people have been making predictions, many using the Drake equation. That involves the use of various educated guesses about the frequency of planets, how many are habitable, and so on. Until about a decade ago, most of the values in the equation remained just that, however: guesses.

In the last dozen years, we've witnessed an amazing transformation in science and appear to be on the verge of several more. The existence of planets orbiting other stars—exoplanets—has gone from a hypothetical to a reality. We've now got a catalog of thousands of potential planets. In many cases, we even have an idea about their size, composition, and temperature. Some of them orbit stars that are, in galactic terms, right next door.

The result has been an incredible buzz of information—over the course of this winter, there were a series of updated estimates on the number of planets in the galaxy (answer: lots) along with various ways of slicing and dicing the numbers. How many Earth-like planets? How many orbiting stars like our Sun? In every case, the numbers were staggeringly large, with the possibility Earth could be one of millions, if not billions, of similar planets in our galaxy alone.

Given all this information, it seems like we're on the verge of finding Earth's twin—a small, rocky, planet sitting at just the right distance from a star to play host to liquid water. But that poses a far more significant challenge than what might be apparent from the field's most recent successes. An understanding of the challenges involved suggests a different time frame, one where we might still be decades away from getting a clear picture of what our galaxy's planets look like.

Spotting exoplanets

There are two main methods we've used for identifying exoplanets: radial velocity and transit. A number of other planets have been spotted through other means, but these two account for the vast majority of sightings.

Radial velocity relies on the fact that gravitational attraction is a two-way street. A star may exert a massive pull on the planets that orbit it but, on a smaller level, those planets pull back. As they swing through their orbits, their gravitational pull tugs the star ever so slightly in the direction of the planet. This motion is enough to cause a slight Doppler shift, the compression or expansion of wavelengths caused as objects move towards or away from an observer. As a star is pulled towards Earth, all of its light gets ever slightly more blue; as it is pulled away, it gets a bit redder.

To track these changes, telescopes have to be fitted with a special instrument called a spectrometer, which tracks how much light is emitted at specific wavelengths. And you have to observe a specific star for an extended period of time to make sure you can spot an acceleration that won't even be visible during portions of its orbit. There are also long- and short-term variations in a star's output (one example is the equivalent of sunspots) that make identifying planets a challenge.

Still, the method is highly effective, and it was the first used to identify an exoplanet. It's especially good if the planet is massive or close to the host star (or both). As a result, many of the first planets we found were what are called "hot Jupiters," gas giants that orbit close in to their host star.

One nice aspect of the radial velocity method is that it provides some indication of the planet's mass, since that determines the strength of its pull on the star. It actually provides a lower limit, since the magnitude of the Doppler effect will be largest if the planet's orbital plane lines up with the line of sight from Earth. You could get the same effect with a heavier planet that has an orbital plane tilted away from Earth.

The alternative is the transit method, which watches for signs of a planet passing between its host star and the Earth. This creates a tell-tale dip in the output from the star, as the planet creates a mini-eclipse. This definitely requires the planet's orbital plane to be lined up so that it runs through Earth. Instead of mass, this method provides an indication of the planet's size, since that determines how much light it can block out.

Combining the two methods gets you both size and mass, which lets you calculate the density of the planet. In multi-planet systems, you can also get this from the transit alone, as the gravitational interactions among the planets can slightly change the timing and duration of the transits.

There are a few other methods used to spot planets, and we've been able to image a number of them directly. But, for the most part, these two account for most of the planets we've observed.

What have we seen?

Initially, there were no instruments designed to detect exoplanets. Instead, researchers had to point existing instruments at a star—and then continue doing so night after night in order to track transits or changes in the velocity of the star. With instrument time at a premium, it's hard to arrange that sort of schedule. So, as a result, the first exoplanets to be spotted were the easiest ones to spot: large, Jupiter class planets orbiting in near their stars. For the first few years of exoplanet hunting, these were the vast majority of the exoplanets spotted.

This raised an obvious question: were we seeing so many just because they were easy to spot, or did this population actually represent the majority of the planets out there? To answer that question, scientists began to design dedicated instruments specifically intended to spot more planets, some ground-based, at least one based in space. In at least one case, a major instrument was fitted with a spectrograph that has specialized in planet hunting. Combined, these instruments began expanding our catalog of exoplanets.

And, in the process, they began to change our picture of our galaxies' planets. Smaller bodies—warm Neptunes and super-Earths—began to appear in the catalog. But we still didn't have a complete enough picture to start making inferences about what the galaxy as a whole looked like.

The Kepler mission was intended to change that, and it has succeeded spectacularly. The space telescope stares down one of the spiral arms of our galaxy, with nearly 150,000 stars in its field of view. Over the past several years, it has found the tell-tale dips in the light that indicates a planet is passing in front of one of them. So far, 105 of these have been confirmed to arise from a planet; there are another 2,740 candidates waiting to be confirmed. We can now start to do statistics.

One of the key things Kepler told us is that most planets are far smaller than Jupiter. First, it became clear there were a lot of Neptune-equivalents, and later, Earth-sized bodies started showing up in the data. It quickly became obvious the numbers went upwards as planet size went down. Rather than being filled with Jupiters, the majority of the planets in our galaxy look much more like Earth. With time, another trend became apparent: the numbers went up the further you got from the star. Not everything was likely to have molten metals bubbling on its surface.

So, that tells us something about the typical planet. How typical are they? Quite. Detecting a planet using Kepler means the system's orbital plane must be edge-on when viewed from Earth. Given the probability of that happening (which is purely a matter of geometry), we can extrapolate out to how many planets must be in Kepler's field of view. And from that, we can estimate how many planets there are in the galaxy total.

The Milky Way contains about 300 billion stars. On average, each of them has a planet (although many of these are in multi-planet systems, meaning many stars have none). Our galaxy is teeming with planets.

Listing image by Aurich Lawson