4,000 Alien Planets Discovered … No, Make That 4,102

4,000 exoplanets. The dense clump that will appear at upper left during the animation is where the Kepler Telescope found most of its planets, since it focused on this specific region of the sky.

The Apollo missions were humankind’s first and so far only opportunity to walk on and explore another planetary body other than Earth. We know of eight planets and countless comets and asteroids in our solar system. The first confirmed detection of exoplanets — planets orbiting other stars — was made in 1992 when two planets were discovered winging around the pulsar PSR 1257+12 2,300 light years in the constellation Virgo. A third was found in the system two years later.

This artist’s view shows the hot Jupiter exoplanet 51 Pegasi b, which orbits a star about 50 light-years from Earth in the northern constellation of Pegasus. A hot Jupiter, the most common kind of exoplanet found, is a Jupiter-sized planet orbiting so close to its host sun it’s heated to high temperatures. M. Kornmesser/Nick Risinger (skysurvey.org)

A pulsar is a tiny, super-dense, city-sized star formed in the aftermath of a supernova explosion. Usually. In this case, it’s thought that two slightly larger stars called white dwarfs merged, with the planets forming from the disk of debris left over from the merger. By 1995 the first extrasolar planet around a sun-like star — 51 Pegasi — was found.

This lively graphic shows the pace of exoplanet discoveries through 2014. The big bump that year came from a slew of new planets discovered by the orbiting Kepler Space Telescope. NASA

Theses discoveries inspired more searches and by 2009 the tally stood at 100 planets. We didn’t strike exoplanet gold until the 2009 launch of NASA’s Kepler Space Telescope, designed specifically to look for exoplanets from orbit above our the blurring effects of Earth’s atmosphere. Five years into the mission in February 2014, NASA announced that Kepler had discovered 715 new planets around 305 stars. The orbiting telescope nabbed its 1,000th planet less than a year later and discoveries have continued apace. Before it was decommissioned in 2018, astronomers Kepler’s tally totaled 2,727 new worlds.

The video-graphic above plots the positions of the planets across a 360° of sky. The fuzzy, looping band is the Milky Way — both the southern and northern hemisphere halves. When it was first posted on July 7, 4,003 alien planets were known. Today, July 25, that number has already swelled to 4,102 orbiting more than 3,000 stars.

Animation showing an extrasolar planet passing in front of a star, causing its light to dim. By studying the light curve (bottom) astronomers can determine planet’s size and other details. Transits of Extrasolar Planets Network

Many approaches have been used to find and confirm planets light years away, but the two most common are the transit and wobble methods. Kepler found its planets by looking for regular, repeating dips in a star’s light when an object passes in front of or transits the star. Knowing the distance and diameter of the star, as well as the length of time the passing object dimmed the star’s light, we can determine its size and mass.

In the radial velocity method, the exoplanet’s gravity causes the host star to wobble back and forth. Measuring the slight change in the color of the star’s light gives information about the planet’s mass and orbit. ESO

In the wobble or radial velocity method astronomers measure the slight wobble of the host star around the planet–star center of gravity caused by the gravitational tug of an orbiting planet. As the star swings toward the observer, the light leaving it shift toward the blue end of the spectrum. As the star moves away, its light is stretched and shifts toward the red end of the spectrum. Measuring these small shifts gives us information about a planet’s orbit and mass.

It’s amazing to watch the video and see the sky fill up with planets. More discoveries lie ahead. NASA’s new Transiting Exoplanet Survey Satellite (TESS), launched last year, has already found some 1,000 new planets with 20,000 discoveries expected during its lifetime. Imagine watching that map fill up with dots. And the more dots the more rife with possibility the universe becomes.