Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!

Bad Astronomy
The entire universe in blog form
Dec. 20 2011 11:18 AM

Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!

Astronomers have achieved a big milestone in the search for another Earth: the two smallest confirmed planets ever found orbiting another star... and they're both about the size of Earth!

Artist's illustration of the Kepler-20 planets with Earth and Venus for size comparison.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


The planets are called Kepler-20e and Kepler-20f, and as you can see by the illustration above they are very close to the same size as our home world: 20e is about 11,100 km (6900 miles) in diameter, and 20f about 13,200 km (8200 miles) across. For comparison, Earth has a diameter of 12,760 km (7930 miles). This makes them the smallest confirmed exoplanets seen orbiting another star! The previous record holder was Kepler-10b, which has a diameter about 40% bigger than Earth's.

To be clear: while these planets are the size of Earth, they are nowhere near Earth-like. The star, Kepler-20, is very much like the Sun, though a bit smaller and cooler (and 950 light years away). However, both planets orbit the star much closer than Earth does; 7.6 million km (4.7 million miles) and 16.6 million km (10.3 million miles), respectively. This is so much closer that both planets must have surface temperatures far hotter than ours, 760°C and 430°C (1400°F and 800°F). Even on the "cooler" planet Kepler-20f, it's hot enough to melt tin and zinc.

So don't start packing your bags to visit, even if you could spare a few million years to get there via rocket (950 light years is a bit of a hike). I'll note that we don't know the masses of these planets either. I'll explain that in a moment, but given their sizes it's expected they'll have masses similar to Earth's.

So this is very exciting! For one thing, it shows that Kepler can indeed find planets the size of Earth orbiting distant stars. That right away is fantastic; that's the main goal of Kepler in the first place.

For another, it shows that our solar system is not entirely unique. We do know of several other stars hosting solar systems of their own, but those planets tend to be very massive; they're easier for us to find. Since Kepler-20e and f are so close to Earth-sized, this is a big achievement.

And we're still not done: there are three other planets in the Kepler-20 system! The others are much larger than the Earth: named Kepler-20b, c, and d, they have diameters of 24,000, 40,000, and 35,000 km (15,000, 24,600, and 22,000 miles); smaller than Uranus and Neptune, but still pretty hefty. We do have the masses for them: 8.7, 16.1, and about 20 times the mass of the Earth. Call them "super-Earths" if you like.

All these planets huddle pretty closely to their star; the orbit of Kepler-20f, the farthest from the star, would still fit comfortably inside the orbit of Mercury! Oddly, the configuration is very different than our own solar system. While ours has the lower-mass planets close in and the bigger ones farther out, in the Kepler-20 system they alternate, going big-little-big-little-big.

So how do we know all this? The Kepler observatory is in space, staring at one patch of sky all the time. There are 100,000 stars in its field of view, including Kepler-20. If there are planets orbiting a star, and we see the orbit of that planet edge-on, then once per orbit the planet directly passes between us and the star, blocking its light a little bit. This is called a transit, and the bigger the planet, the more light it blocks. That's how the sizes of the five planets were found.

As these planets orbit their star their gravity tugs on it, and that can be measured by carefully observing the star's light. As a planet pulls it one way and then another, there is a very small Doppler shift in the starlight, and the amount of that shift tells us how hard the planet is tugging on the star, and that in turn depends on the mass of the planet. Only the three bigger planets in the Kepler-20 system pull hard enough for us to measure, which is why we don't have the masses of 20e and 20f; they're too small to measure.

Also, to be clear, we don't have direct images of these planets (those pictures above are drawings). They were found indirectly by how they affected their star. But these methods are now tried-and-true, and the existence of these five planets has been confirmed. They're real.

This is a fantastic discovery for so many reasons: the smallest planets found orbiting another star, the first Earth-sized planets seen by Kepler, both in the same solar system, and in such an oddly-configured and compact system at that. This means we need to think more about how such planets can form, of course, since it's so weird... but no matter what, it means we're that much closer to finding the ultimate goal: an Earth-sized planet orbiting a Sun-like star in that star's habitable zone, where liquid water can exist.

Every time I hear news like this, I wonder how much longer we'll be waiting to hear that news... and I strongly suspect it won't be too much longer.

Image credit: NASA/Ames/JPL-Caltech

Related posts:



Smash and Grab

Will competitive Senate contests in Kansas and South Dakota lead to more late-breaking races in future elections?

Stop Panicking. America Is Now in Very Good Shape to Respond to the Ebola Crisis.

The 2014 Kansas City Royals Show the Value of Building a Mediocre Baseball Team

The GOP Won’t Win Any Black Votes With Its New “Willie Horton” Ad

Sleater-Kinney Was Once America’s Best Rock Band

Can it be again?


Forget Oculus Rift

This $25 cardboard box turns your phone into an incredibly fun virtual reality experience.

One of Putin’s Favorite Oligarchs Wants to Start an Orthodox Christian Fox News

These Companies in Japan Are More Than 1,000 Years Old

Trending News Channel
Oct. 20 2014 6:17 PM Watch Flashes of Lightning Created in a Lab  
  News & Politics
The World
Oct. 20 2014 1:50 PM Why We Shouldn’t Be Too Sure About the Supposed Deal to Return the Abducted Nigerian Schoolgirls
Oct. 20 2014 7:23 PM Chipotle’s Magical Burrito Empire Keeps Growing, Might Be Slowing
Dear Prudence
Oct. 20 2014 3:12 PM Terror Next Door Prudie advises a letter writer whose husband is dangerously, violently obsessed with the neighbors.
  Double X
The XX Factor
Oct. 20 2014 1:10 PM Women Are Still Losing Jobs for Getting Pregnant
  Slate Plus
Tv Club
Oct. 20 2014 7:15 AM The Slate Doctor Who Podcast: Episode 9 A spoiler-filled discussion of "Flatline."
Brow Beat
Oct. 20 2014 9:13 PM The Smart, Talented, and Utterly Hilarious Leslie Jones Is SNL’s Newest Cast Member
Future Tense
Oct. 20 2014 4:59 PM Canadian Town Cancels Outdoor Halloween Because Polar Bears
  Health & Science
Medical Examiner
Oct. 20 2014 11:46 AM Is Anybody Watching My Do-Gooding? The difference between being a hero and being an altruist.
Sports Nut
Oct. 20 2014 5:09 PM Keepaway, on Three. Ready—Break! On his record-breaking touchdown pass, Peyton Manning couldn’t even leave the celebration to chance.