A long time ago something very catastrophic happened in the Kepler-107 system.
Kepler-107 is a star of about 1,700 light-years. It is a bit similar to the Sun, though a bit more massive (1.2X), larger (1.45X) and warmer (by about 80 ° C) than our own star. It's just a little younger at 4.3 billion years, compared to 4.6 for the Sun.
There are planets, at least four of them. They were discovered by Kepler's satellite, which was looking for dips in the starlight if a planet happened to pass in front of the star once for orbit, as seen on Earth (we call this transit and wrote them extensively as discussed in my astronomy episode of Crash Course: Exoplanets).
This is where the similarities with our solar system are over. The four known exoplanets circling around Kepler-107 are very close to their star compared to our planets. Called Kepler 107-b through e (to get out of the star), they are all a little closer to the star from Mercury to the Sun. The nearest is less than 7 million kilometers, giving it a hot surface temperature of about 1300 ° C, hot enough to melt iron! It's not exactly a garden place.
The dimensions of the planets can be determined by how much light the stars get immersed in each transit. The outermost, 107e, is the largest about 3 times the diameter of the Earth. 107 d is the smallest at 0.9 Earth. 107b and c are approximately the same size, at 1.54 and 1.6 times in our own home.
What about their tables? Well, here things get bizarre.
The best way to get the mass of a planet is to see how hard it pulls on its star as it rotates around it. The star has a lot of mass, which means a lot of gravity to draw on the planet. But the planets also have a mass, so they also pull the star. Since the planet makes a large circle (or ellipse) around the star, the star does a lot less – technically, they both move around the center of the table, called the barycenter. If you know the star's mass, and how much the planet pulls it, you can get the planet's table (I'm talking about it again in the Crash Course's astronomy episode at 1:29 am).
Astronomers used a very complicated camera to do just that, and what they got was a big surprise. Although the two innermost planets Kepler 107b and c are of the same size, 107c is much more massive. 107b has a mass around 3.5 times Earth, but 107c has a mass of 9.4 times Earth!
If there is a larger table but the same size, it should be much thicker. While 107b has a density approximately equal to the Earth's (about 5.3 grams per cubic centimeter, also equivalent to 5.3 times the water density), 107c has a density greater than twice as much as 12.7 grams per cubic centimeter! This makes it one of the densest ever found planets.
Considering what we know about the formation of planets, it is likely that 107c has an iron core similar to Earth, but the core is so huge that it is 70% of the planet's total mass! This is more than twice the Earth's massive mass of the Earth. Around the core of 107c is probably a rocky robe and bark, which makes the other 30%.
How has it become so tight?
Sometimes, when the planets circle around their star, the warmth and light of the star can literally blow the atmosphere of the planet. If you start with a gas giant, after a while the atmosphere with lower density is removed, leaving behind the thicker core. However, you would expect this to be a more serious problem for the innermost planet, and 107b is less than 107c. So this can not be.
Enter the planetary apocalypse.
I do not exaggerate. Another way to remove the lighter things from a planet is to crush it with another planet.
Astronomers who examine this conduct some computer models of what happens when the planets collide and find that the density and locked internal structure of 107c can be explained if two planets, each about ten times larger mass of the Earth, collided billions of years ago,
You know, allow me to reformulate this: Astronomers who study this have done some computer models of what happens when the planets collide and find that the density and the internal structure of 107c can be explained if two planets each of ten times the MASS OF EARTH COLLECTED MAIN BUILDINGS OF THE YEARS OF AIE!
This has a great impact. Similarly, literally the destruction of the planet. More than half of the mass of the two planets would crumble or evaporate from the energy of impact and be thrown out – most of it consists of rocky material with lower density – leaving the thickest things like iron to merge and form the new, denser planet.
This idea is not unprecedented. In our solar system, Mercury has an unusually large core, and perhaps has once been a larger planet that has undergone such a collision. We believe that the Earth has also dealt with, and the debris of this effect forms the moon.
However, given the huge size and mass of 107c, it was a hilly wreath much larger than what had crashed the Earth or Mercury.
Now, this is not written in stone (nor iron). But, as I said, collisions happen quite often, so I think a huge planetary catastrophe is the way to bet here.
And it makes me wonder how often and how stormy these clashes are. The only way to find out is to find more planets, more planetary systems, and see how they behave. It's incredible to me that this is where we are in exoplanet science, given that the first one was discovered less than 30 years ago! And now we've found so much that we can start looking for trends, stories, resemblances, and differences with our own solar system.
Again and again we find we are weird here. It is fun to me that the more planets we find, the more I wonder why we thought differently.