How many planets are out there?
Beyond the eight planets of our own solar system are another 3,500 planets that we know of, and probably millions or billions more yet to discover. The first exoplanet* was spotted in 1989, but because the very existence of planets orbiting other stars was still a radical theory, its discoverer, David Latham, referred to it only as the “companion” of its star, HD 114762, and its planetary identity wasn't confirmed until 2012. Because of the delay in confirmation, most of the “first exoplanet” attention went to 51 Pegasi b, the first planet discovered around a main-sequence star, in 1995.
In the past few years, exoplanet discovery has become so common that between one and 10 planets are added to NASA's official Exoplanet Archive nearly every week, says Jessie Christiansen, an astronomer who has worked with the NASA Exoplanet Science Institute for 4 years. “Sometimes there will be a week with none, sometimes there will be a week with 12,” she adds.
What are they like?
As you might imagine, exoplanets vary radically in size, orbit, rotation, and composition – and much more than Star Trek might have led you to believe. Many are rocky worlds like Earth and Mars, while others are are gassy giants like Jupiter or Neptune. They range in size from tiny Kepler 37 b, about the size of Earth's moon, up to the gigantic HD 100546 b, which is almost half a million times the volume of Earth.
The closest of the exoplanets we've found is Proxima b, which is about four light-years away. That's far beyond the reach of existing satellites, but some engineers are working on solar sail technology that could theoretically carry a tiny “nanosatellite” there in about 20 years.
For years, astronomers only found single-planet systems – planets alone with their star – because those were the easiest to detect. But as exoplanet-hunting technology has improved, astronomers are finding more and more star systems – the current count is 582 – populated with anywhere from 2 to 7 planets each.
(Our own system, with 8 planets plus dwarf planets like Pluto, is still the most populous one we know of.)
Compact solar systems with multiple small planets have “really become ubiquitous,” says Dr. Christiansen, "and possibly even the norm.”
How do astronomers find them?
Until 2013, most of the exoplanets were found via radial velocity, also known as the “wobble” method. Every particle in space exerts a pull on every other particle, and the bigger and closer you are, the stronger your gravitational pull. Planets and stars tug on each other, and as a planet goes around a star, it makes the star wobble just a bit – and with sensitive enough instruments, we can spot those wobbles and use them to calculate the planet's mass.
Kepler space telescope began hunting planets in 2009. It specifically set out to find Earthlike planets transiting stars, and it has already found more than 2000 of those. A transit occurs when one body (like a planet) crosses in front of another (like its star). If they are exactly the same apparent size, the transit is called an eclipse, but more often, a transit shows up as a brief dimming in a star's brightness.
Unfortunately, transits show up only if the planet and star are exactly in line as they appear from our telescopes on Earth. If the planet orbits in any other plane, it won't change the star's brightness and so we won't see it. It will, however, affect the orbit of any other planets in its solar system. Since 2011, 15 planets have been discovered just that way – by noting slight lags in the orbit of a known, transiting planet that can be explained only by another planet nearby.
Where is 'Earth 2.0'?
Many exoplanets have been given that nickname over the years, usually because of some key similarity each one shared with our home planet. Several confirmed exoplanets have almost exactly the same overall density as Earth, five are the same size as Earth (and another 100 are within 10 percent of Earth's size), and three are close to Earth's mass.
But so far, none has clear evidence of flowing liquid water, let alone life – two traits that, so far, still appear unique to our rocky little blue planet.
[Editor's note: Precise credit for the first discovery of an exoplanet is an area of debate among scientists, as NASA explains here.]