NASA’s Kepler space telescope is busted and it may never work normally again. But during its four years of exemplary service, the planet-hunting telescope provided astronomers with an unprecedented glimpse into the Milky Way. Here are the most incredible discoveries made by Kepler.

Top image: Ron Miller.

As we reported yesterday, controllers on the ground can no longer control Kepler’s orientation. Two of its four reaction wheels have broken down and it’s unlikely that it can be repaired. That said, the Kepler team hasn’t given up hope, saying there may be some ways to revive the telescope.

Regardless, the space telescope has done its bit for king-and-country, operating for six months longer than initially planned.

Over the course of its three-and-half-years, the $600 million Kepler mission helped astronomers identify 132 exoplanets, including another 2,700 candidates. But it’s also provided us with a clearer picture of the Milky Way — a galaxy that contains bizarre worlds, freaky solar systems, and an astounding number of potentially habitable planets.


Here’s a rundown of Kepler's most significant and jaw-dropping discoveries.

The First Confirmed Discovery of a Rocky Planet

In January 2011, astronomers discovered Kepler-10b — the most Earth-like planet known at the time. It marked the first time in history that a terrestrial planet was discovered outside of our own solar system. Kepler scientist Douglas Hudgins put it all into perspective:

The discovery of Kepler 10-b is a significant milestone in the search for planets similar to our own. Although this planet is not in the habitable zone, the exciting find showcases the kinds of discoveries made possible by the mission and the promise of many more to come.


The First Confirmed Discovery of a Potentially Habitable Planet

It’s one thing to find a rocky planet, but quite another to find one orbiting within its solar system’s habitable zone. Astronomers have now catalogued a handful of these potentially habitable exoplanets, but the first to ever be discovered was Kepler-22b. It has a radius 2.4 times that of our planet and it orbits about 15% closer to its star than Earth does to the Sun.


It’s also significantly cooler, dimmer, and smaller than ours. And while scientists have yet to determine K-22b's composition — be it rocky, gaseous or liquid — they estimate that surface temperatures on K-22b average a very Earth-like 72-degrees Fahrenheit.

Image: NASA/Ames/JPL.

Evidence that 17 Billion Earth-Sized Planets Exist in Our Galaxy

Back in January, Kepler data helped astronomers estimate the number of Earth-sized planets in the Milky Way. They surveyed about 2,400 candidate planets spotted by the Kepler satellite over the first 16 months of its operation. The data indicated that about 17 percent of stars have an Earth-sized planet in an orbit closer than Mercury — that's about one in every six star systems. Given that the Milky Way has about 100 billion stars, that adds up to the figure of 17 billion. If there are this many Earth-sized planets in Mercury-like orbits, it's probably safe to assume that there's a substantial number residing further out in the habitable areas.


Image via Harvard-Smithsonian Center for Astrophysics.

Evidence that 4.5 Billion Potentially Habitable Planets May Orbit Red Dwarfs in Our Galaxy

Red dwarfs comprise nearly 75% of all the stars in the Milky Way, yet they remain invisible to the naked eye.


Using data pulled from Kepler, astronomers now believe that as many as 6% of all red dwarfs host Earth-sized planets within their habitable zones — a calculation that brings the total number of red dwarf alien Earths across the galaxy to 4.5 billion.


Image: Artist's impression of a sunset from the super-Earth Gliese 667Cc courtesy ESO/L.Calçada. The large sun is the red dwarf, 667C.

The First Confirmed Discovery of a Tatooine-Like Planet

On September 15th, 2011, NASA discovered a "circumbinary planet," orbiting not one, but two stars — just like Tatooine.


Located 200 light years away, Kepler-16b is a binary star system that hosts a Saturn-like planet composed of both gas and rock. The larger of the two suns is roughly 69% the mass of our Sun, while the smaller, red star is closer to 20% of our Sun's mass. Kepler also discovered the first binary system with more than one planet. Others have since been discovered.


Image: NASA/JPL-Caltech.

The First Confirmed Discovery of a Planet with Four Suns

Kepler data was also made available to the general public. Back in October 2012, two amateur astronomers confirmed the existence of a Neptune-like planet with four suns, designating it the first quadruple star system ever discovered.


The planet, which is 5,000 light years away from Earth, closely orbits one pair of stars, which in turn forms a unit that revolves around a second pair at a distance of around 1,000 AU.


Image: Ron Miller.

The Discovery of a New Type of Super-Dense Planet

Several years ago, Kepler scientists discovered celestial bodies that appeared too heavy for their size.


The going theory is that they’re an entirely new class of planet — Neptune-like planets that were stripped of their outer gaseous layers after venturing too close to their sun. Once ice giants, these planets migrated inwards — as their orbits were affected by interactions with surrounding gas and dust — perhaps getting as close to their stars as Mercury is to ours.


Early last year, scientists also confirmed the discovery of a new class of exoplanet called a boiling waterworld.

Kepler has also found strange new solar systems that defy classification, like the one featuring a Neptune-like planet locked in a close orbit with a terrestrial planet.


Artistic impression of Kepler-36 by David Aguilar, Harvard-Smithsonian Center for Astrophysics.

The Discovery of a Planet Getting Ripped to Shreds by Its Sun

Kepler helped astronomers find a short-period super-Mercury planet that is in the final stage of its life.


The object has gotten so close to its parent star that it's only taking 15.7 hours to orbit around it, while it's surface temperature has risen to 3,600 degrees Fahrenheit. The result: a dramatic, comet-like tail that's bursting outward from the planet — and with it, much of the planet's surface.


Image via NASA/JPL-Caltech.

The Most Earth-Like Planets Ever Discovered

This past April, the Kepler mission discovered a trio of distant planets that look remarkably like our own.


Three Earth-sized worlds, dubbed Kepler-62f, Kepler-62e, and Kepler-69c, were discovered 1,200 and 2,700 light years from Earth — and they’re all situated in the so-called Goldilocks Zone of their parent stars, the "just-right" range at which liquid water, and life, can exist on a planet's surface.


The Development of a New Planet-Hunting Technique

Just announced this past week, data from Kepler helped astronomers find a hot Jupiter that’s 2,000 light-years away — and they did so using a new technique called BEER (relativistic BEaming, Ellipsoidal, and Reflection/emission modulations). The new planet-finding technique exploits an effect predicted by Einstein’s Special Theory of Relativity. Called beaming, it happens when a star’s brightness is increased as it moves towards the Earth, and dims as it moves away. It moves toward us because a planet is there to pull it (hence evidence of its presence). The brightening is caused by light particles, called photons, that are piling up in energy. This technique can now be used in conjunction with the wobble method and the transit method.