It’s official: you are weird. It’s nothing personal, you can’t help it. You live in a weird Solar System. It’s just not like other planetary systems. It’s just not SUPER enough….
Most Sun-like stars have a planet orbiting close-by. Not a hot Jupiter — those are rare. Planets just a little bigger than Earth are much more common. Between about one and four times the size of Earth. These kind of planets are often called “super-Earths”. Not because they are inhabited by super-people (to my knowledge). Just because they are a little bigger than Earth. What is interesting is that super-Earths are everywhere!
The image below shows how common planets of different sizes are (thanks to Kepler). These planets all orbit very close to their stars. Most of these planets’ orbits are smaller than Mercury’s.
There are a ton of planets out there! Take 100 stars like the Sun. Forty stars have a planet a little larger than Earth (1.25 to 4 times Earth’s size). Another 17 have a planet the same size as Earth. About 3-5 have a bigger (gas giant) planet. When you add it up, more than half of the 100 stars have a planet close to their star. But we don’t. The Solar System’s closest planet is Mercury. Mercury is so tiny (just 38% as big as Earth) that it doesn’t even show up on the plot.
This is why the Solar System is weird! Or at least in the minority. It’s not because of the planets we have but because of the planets we don’t. Imagine a man who grew up stranded on a deserted island. He spent years learning to carve a seaworthy boat from native timber. He set off on a quest to find other people. He spent weeks and months alone on the ocean. At last he landed on a beach. The beach was full of people, most of whom were wearing tight black leather pants. How could he ever have expected most people to wear tight black leather pants? But that’s how it was on that beach. Our Solar System is like that castaway. We have no big close-in planets so we certainly didn’t expect them to be so common. But that is what we see. Of course, what we see is what we can see: big planets close to their stars. And from just that limited sample we can tell that we are different. [Personal note: I don’t recommend tight black leather pants at the beach]. In some sense, we already knew that the Solar System is weird because the Sun is not an average star! But I digress…
Back to reality. Just last week the Kepler mission announced the discovery of another 715 extra-solar planets! Most of these are again 1-4 times the size of Earth. More super-Earths and mini-Neptunes!
Super-Earths? Mini-Neptunes? What are these things? The Solar System doesn’t contain any planets between 1 (Earth) and 4 (Neptune) times the size of Earth. So it’s not obvious whether planets in this range are more likely to be scaled-up versions of Earth (“super-Earths”) or scaled-down versions of Neptune (“mini-Neptunes”). How can we tell super-Earths and mini-Neptunes apart?
The simplest way is to measure the planets’ densities. Earth’s density is 5.5 grams per cubic centimeter. Venus, Mercury and Mars all have densities between 3.9 and 5.5. They are made mostly of rock and iron. Neptune’s density is 1.6. Uranus’ is 1.3. Uranus and Neptune are made of ice and gas (and some rock). So mini-Neptunes should have low densities and super-Earths should have large densities. This makes sense because rock is heavier than ice or gas. The problem is that it’s very hard to measure a planet’s density! Especially for such small planets. This is just now being measured for enough planets to test these ideas.
This figure shows the densities of extra-solar planets of different sizes. Each measurement (black dot) has an associated measurement error (black line). You can tell how hard densities are to determine because the error bars are really big for the smallest planets.
Planets that are just a little bigger than Earth tend to be even denser than Earth. These are super-Earths! They are denser than Earth in part because these planets are compressed under their own gravity. Larger planets turn around and become steadily less dense. These are mini-Neptunes. They are less dense because they contain gases that puff them up.
Dense super-Earths only seem to exist at small sizes. Planets larger than 1.5-2 times Earth are mini-Neptunes. The compositions of planets therefore makes the same transition. The following graphic shows a slice through planets spanning the transition from super-Earths to mini-Neptunes.
Small planets are made of the same stuff as Earth: iron and rock with a little water thrown in. Bigger planets are different. Most of their bulk is in the form of gas. Just a few percent contribution of gas can double a planet’s size! Planets that are bigger than a certain size have gas but small ones don’t. The bigger the planet, the more gas. Farther down this path are gas giants like Saturn and Jupiter, planets that are completely dominated by gas.
We can understand this progression in the context of planet formation. In the gaseous planet-forming disk, only large planets have enough gravity to hold onto a thick atmosphere. The bigger then planet, the thicker the planet’s atmosphere. If a planet becomes big enough it enters a feedback loop and can grow like crazy. First, the planet’s gravity increases and it can hold onto a little more gas. Next, it grabs a little more gas from its surroundings. This increases its gravity and it starts over again. This loop is how a Neptune-sized planet can quickly grow into a Jupiter-sized one.
This is kind of like how people get rich. Think of bigger planets as being richer. When you’re poor you have no money to invest. This is like Earth and the super-Earths. When you have more money, you can invest some of it and (if all goes well) you will open up a whole new source of income. This is like mini-Neptunes (and the real one too). Once you have enough money, you can leverage it to make a ton more money very quickly. This is like Jupiter and Saturn. The poor super-Earths are just a little too short on cash to invest and gain access to a whole new source of income.
The planetary systems that contain super-Earths or mini-Neptunes are also different than the Solar System. First of all, the planets’ orbits are much closer to their stars. That is why they were discovered — the closer to their stars, the easier to find. Second, these planets are almost always found in systems with several other super-Earths/mini-Neptunes. The orbits in these systems are closely-packed. A lot more planets are squashed into the same amount of space than in the Solar System. In terms of the orbital dynamics the systems are similar to the terrestrial planets in the Solar System. Their orbits are smaller but their interactions are roughly the same.
Below is a schematic of the Kepler-62 system. It is an especially interesting 5-planet system because it has two planets in the habitable zone. These planets are 1.4 and 1.6 times larger than Earth so they are likely to be super-Earths rather than mini-Neptunes.
Here are a few highlights of the newly-discovered systems of super-Earths and mini-Neptunes. The Kepler-11 system has six planets including at least five mini-Neptunes. The Kepler-37 system contains three planets including two that are smaller than Earth. The Gliese 581 system contains four confirmed planets including one that under certain atmospheric conditions can have liquid water on its surface. There are a lot more fascinating systems out there. Check out the Kepler mission website here.
How do systems of hot super-Earths or mini-Neptunes form? I’m glad you asked! This is something I’ve spent a lot of time thinking about. In fact, I’ve spent so much time thinking about this that I’ll go into detail in a future post. Next week: gas giant planets on wide but weird-shaped orbits.