First Exoplanet Smaller Than Earth: Why I'm Not Surprised

"Continue to surprise those who would put you in a neat demographic. Be insistently curious." -Gordon Gee

Twenty years ago, our Solar System was the only one we knew of that we were certain had planets orbiting around a main-sequence star.

Image credit: retrieved from Universe-Review.ca.

Perhaps surprisingly, it wasn't until 1995 that the first Extra-Solar Planet (exoplanet) -- or planet orbiting a star outside of our Solar System -- was discovered. And when it was, it was nothing like the planets in our Solar System. In fact, most of the earliest exoplanets discovered were not only more massive than Jupiter, but orbited their parent star even closer than Mercury orbits out Sun!

Image credit: Fran Bagenal at University of Colorado.

Immediately, there were astronomers (and many non-astronomers) who speculated that these Hot Jupiters were very common, and that Solar Systems like our own were perhaps rarities.

And if this sends alarm bells ringing in your head as the dumbest thing you've ever heard, I've got news for you: you're right, and you are already smarter than everyone who thought this. Here's why.

Image credit: Exoplanets.org.

The method by which the first exoplanets were ever detected was through a phenomenon known as stellar wobble. You know that the planets orbit the Sun in ellipses, due to the laws of gravity. Well, technically, all the planets and the Sun orbit their mutual center of mass, which we approximate as being, well, the center of the Sun.

This is reasonable, because the Sun has 99.8% the mass of the Solar System! But in reality, the Sun wobbles back-and-forth in its own tiny ellipse, orbiting the center of mass that is primarily defined by Jupiter, but is still affected by the combined gravitational influence of all the planets, moons, comets and asteroids in our Solar System. So how would you detect this motion from a different Solar System?

Image credit: Sally Bensusen/Science Photo Library.

When the star moves towards you in its orbit, the light from it becomes slightly blueshifted due to its motion towards you, and when it moves away from you, it gets redshifted an equal amount. The faster the motion, the greater the shift.

So what sort of exoplanets are you most likely to detect using this method? The ones that are most massive and closest to their star, because the closer they are, the more quickly they orbit!

In other words, we didn't find these "Hot Jupiters" because they're so common and we're so rare, we found them because those are the easiest things to see! In fact, the idea that we see more of the things that are easier to see has been around in astronomy since 1922, and is known as Malmquist bias. Let's show you what that is.

Image credit: Wikipedia user Galaxy1F10.

Imagine you look out at the Universe, and it's full of stars with all sorts of brightnesses -- very bright stars and very dim stars -- both near to us, at intermediate distances, and very far away. Close to us, perhaps our eyes (or telescopes) are good enough that we can find all the stars. In other words, everything above the red, solid line in the image above. But what do you see when you look at a star cluster -- like Messier 103 -- that's located very far away?

Image credit: Hillary Mathis & N. A. Sharp, KPNO 2.1-m Tel., AURA, NOAO, NSF.

You only see intrinsically bright stars! "Maybe there only are intrinsically bright stars at large distances," say the people with no imaginations. But Gunnar Malmquist knew better. All the stars are there -- bright ones and dim ones -- but you only see the bright ones because that's all you're sensitive to!

It's incredibly obvious in hindsight, and it means that once you build a telescope that's more sensitive to light, you can see fainter objects at farther distances. This concept of Malmquist bias is also applicable to all sorts of observations and situations in astronomy.

Image credit: Sam Pitts, of the Hercules Cluster Abell 2151.

Even looking out at a distant galaxy cluster, located nearly a billion light years away, you shouldn't be surprised that the galaxies you see are bigger and brighter than nearly all the galaxies in our local group! A very deep image like the one above (with an exposure time of over four hours) reveals fainter, less impressive galaxies, and in greater numbers than the brightest, most easily visible ones.

That's the lesson you should take away from this: you see the kinds of objects your instrument is designed to see. But just because what you're seeing far away looks different from what's close by doesn't mean it is different from what's close by. So fast forward to today, where we've got a much better, more successful way to find exoplanets than by this "primitive" wobble method.

Image credit: NASA / Tim Pyle, retrieved from GizMag.

Using the transit method, our most sophisticated planet-finding spacecraft, Kepler, has found thousands of planets, compared to the dozens that were found with the wobble method. When an exoplanet passes in between our line-of-sight and its parent star, it blocks some portion of the star's light. This temporary "dip" in the brightness of a distant star is how we can detect a planetary transit, and hence infer the existence of an exoplanet.

Image credit: Поташев Роман Евгеньевич, of exoplanet Kepler-6b.

So, think about it for a minute: what types of planets will we be most likely to see? Which ones will be the easiest to see and verify? Well, that would be

• the biggest ones, because they'll block the most light and be the most noticeable,
• the innermost ones, because they'll be most likely to transit in our line-of-sight to the star, and
• the ones that orbit the fastest, because it takes multiple transits to confirm that this is, in fact, an exoplanet rather than just a rogue object or stellar fluctuation.
In other words, the types of planets it's most likely to find are large inner planets: super-Earths!

Image credit: NASA / Ames / JPL-Caltech.

The recent discovery of Kepler-22b, with an Earth-like orbit around a Sun-like star, was exactly this; it's estimated that Kepler-22b is more than 10 times the mass of Earth! In fact, the vast majority of planets found by Kepler are these super-Earths, with not a single planet found that was Earth-sized or smaller.

You can guess what's coming, can't you? Cue Physics Today, with the headline,

Super-Earths give theorists a super headache.

Any guesses, mind you, as to what the theoretical limit of how small a planet Kepler could possibly detect, at the very limit of its power?

Image credit: NASA / Ames / JPL-Caltech.

Did you guess something just barely smaller than Earth, and only then if it's mind-bogglingly close to a star that's significantly smaller than our Sun?

Good guess! And guess what NASA just announced earlier today?

Image credit: NASA / Ames / JPL-Caltech / Tim Pyle.

Say hello to Kepler-20e, the first exoplanet ever discovered that's smaller than Earth! (In fact, it's even smaller than Venus.) With a mass far too small to be detected by stellar wobble, Kepler-20e is only detectable via this transit method! And we're lucky Kepler's been watching this star so closely; Kepler-20 five discovered planets well inside the orbit of where our Mercury resides!

Image credit: David A. Aguilar (CfA).

The larger planets meant that Kepler-20 was already a star of interest, and that many orbits of these planets around their stars were carefully watched. Each time a planet passes in front of its star, it blocks its light. The longer you watch it, the more sensitive you become to small but recurring light-blocking signals.

And that's exactly what we've got here: Kepler-20e, the smallest one, whips around its star every 6.1 days, while it takes Mercury -- our Solar System's speedster -- a comparatively lackadaisical 88 days. Even the fourth planet from this star, Kepler-20f, makes a complete orbit in 19.5 days. Kepler-20e is smaller than Venus, while Kepler-20f is only 3% larger than Earth! The result of all this careful watching is a Solar System where the inner five planets are all scorchingly unsuitable for life, but have a very rich orbital tale to tell.

Image credit: NASA/Ames/JPL-Caltech, retrieved from Ian O'Neill.

And just like that, the delusional "super-Earth mystery" should be smashed to bits!

So don't be surprised at all the super-Earths so far, the smaller planets are just harder to see, and we're only starting to get there. By time the next generation of planet-finding telescope comes along, we're going to be rolling in Earths and mini-Earths, just you wait!

For more on this find, check out Phil Plait's, Nancy Atkinson's, and even the Telegraph's take, and for all the latest exoplanet news as it comes in, I highly recommend this collection. Kepler-20e: it's the first exoplanet we've ever found that's smaller than Earth, but it won't be the last!