The Great Planet Bake Off

Title: Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars

Authors: R. Luque, E. Pallé

First Author Institution: Instituto de Astrofísica de Andalucía (Consejo Superior de Investigaciones Científicas), Granada 18008, Spain

Status: Published in Science, [closed access]

Imagine every cookie you have ever eaten. Yum. They were each a little different from each other, but some probably were quite similar. They might have been made with a similar recipe or a similar baking strategy. Even though you might not have been there when they were made, if you can group similar ones together (sweet ones, crispy ones, buttery ones) you will have more information about how a baker goes about making cookies in general. That’s what today’s authors did, not with bakers making cookies, but stars making planets!

Here are some we prepared earlier

While we have no evidence yet of stars making cookies, they are quite good at making planets out of the disks of material that are left over when they form. How planets form is a major question in the exo-planet field, because just like with the cookies, we don’t often get to see it happen. Disks can be challenging to observe, plus the process takes tens of millions of years and who has that kind of time anymore. So we are left trying to understand the process by looking at the results.

Today’s authors looked at a small set of about 50 of all the 5,000+ known exo-planets. They only looked at ones that had been found using the transit method, and then followed up with ground based observations, and that were found around small M stars. This was important because by combining these kinds of observations they could get mass and radius estimates, which are generally more accurate when the host star is an M dwarf. This is due to the fact that the same sized planet will block a larger percentage of light/have a greater gravitational effect on a smaller star compared to a bigger one, and therefore be easier/more accurate to measure.

The taste test

They weren’t looking at the butteriness or crispiness of the planets, but their bulk composition, or what kind of ingredients were being used to make the planet. If the planet is mostly rocky like our Earth, it would be sized for a given mass. But if the planet is half water and half rock, the extra water will make the planet appear much puffier. For comparison, all the water on Earth is about 0.05 % of the planet’s mass (96% of it on the surface in the oceans), so these would truly be water worlds! 

Figure 1) The mass and radius of each of the 50 rocky planets found around M stars, color coded by their temperature. Two major populations stand out, one that traces an Earth-like, fully rocky compositions (green line), and one that follows half-water half-rocky worlds (blue line). Some planets are puffier than either model and might have gassy atmospheres.

With their pile of “cookies” assembled, the authors plot the planets’ masses versus their radii, to get an idea of how puffy they are which will tell them about the bulk composition. And that’s just what they did in Figure 1! They even added two lines that model the rocky and half water/half rock “planet recipes”. Almost all of the planets fall nicely onto one of the two lines. Planets that fall on the rocky recipe line have a wider range of masses while half water/half rock have a minimum mass of 2-3 earth masses.

It’s also worth noting that a few planets have a larger radius than expected for their mass and aren’t explained by either recipe. There are recipes that do explain them, rocky planets with big gassy atmosphere, or watery ones that have puffy atmospheres due to a greenhouse effect (speaking of which, did you know Astrobites has a climate change series?).

And the winner is…

So what can these recipes tell us about the baking process for planets? Planets should form from a combination of ice and rock combining together in the protoplanetary disk of the star. There are two main theories on how this happens. Both start with a planet core forming, but it then grows thanks to either planetesimal accretion (km sized material collide and get mixed in) or pebble accretion (cm sized material accumulates over time). Since there seem to be no planets in between the full rocky and the half water/half rock recipes, this would point to pebble accretion, as planetesimal accretion predicts there to be some objects with intermediate composition. This would then mean that “baking strategy” for all the stars is the same, and the difference in composition for their planets is where they are made in the protoplanetary disk, the rocky planets forming within the iceline (a distance far enough from the host star beyond which ice can exist), and the half water/half rock form beyond and potentially migrate inwards later. For the multi planet systems in their sample, the rocky planets are always the interior ones and the other ones are water worlds, so it seems to fit!

While this result only used planets around M dwarfs, the authors do speculate about stars that are more massive (such as our Sun). They point out that theoretical models predict similar results, and simulations using planet formation and evolution models support their findings. However there aren’t enough well constrained planet radii to do the same analysis for F-,G-,and K- type stars. Still, this is a huge step in understanding planet formation, and a very tasty result!

Astrobite edited by Graham Doskoch

Featured image credit: wikipedia and NASA/JPL-Caltech

About Mark Popinchalk

I'm a postdoc at the American Museum of Natural History. I study the age of stars by measuring how quickly they rotate. I enjoy ultimate frisbee, baking bread, and all kinds of games. My favorite color is sky-blue-pink.

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