Title: Density, not radius, separates rocky, water-rich minor planets orbiting dwarf stars M
Authors: R. Luque, E. Pallé
First Author Foundation: Instituto de Astrofísica de Andalucía (Consejo Superior de Investigaciones Científicas), Granada 18008, Spain
condition: Posted in Science, [closed access]
Imagine every cookie you’ve ever eaten. yum. They were slightly different from each other, but perhaps some of them were quite similar. They may have been made with a similar recipe or a similar baking strategy. Although you probably weren’t there when they were made, if you can group the likes together (sweet, crunchy, buttery) you’ll get more information about how the baker makes cookies in general. That’s what today’s authors did, not with bakers making cakes, but stars making planets!
Here are some we prepared earlier
Although there is no evidence yet that stars make cookies, they are very good at making planets from disks of material left over when they formed. How planets form is a major question in the exoplanet field, because just like with cookies, we don’t see that happen very often. Observing discs can be tricky, plus the process takes tens of millions of years and who has that kind of time anymore. So we were left trying to understand the process by looking at the results.
Today’s authors looked at a small group of about 50 of all the 5,000+ known exoplanets. They only looked at those found using the transit method, and then followed them up with ground-based observations, which were found around young M stars. This was important because by combining these types of observations, they could get estimates of mass and radius, which are It is generally more accurate when the host star is an M dwarf. This is due to the fact that a planet of the same size will block a greater proportion of light/have a greater gravitational effect on a smaller star than on a larger star, and thus be easier/more accurate to measure.
They were not looking at the bowl or the fragility of the planets, but they were looking at their massive composition, or what kind of ingredients were used to make the planet. If a planet is mostly rocky like Earth, it will be of adequate size for a given mass. But if the planet is half water and half rock, the extra water will make the planet appear more puffy. For comparison, all the water on Earth makes up about 0.05% of the planet’s mass (96% of it is on the surface in the oceans), so these would really be watery worlds!
Assembling a stack of “cookies,” the authors plotted the masses of the planets against their radii, to get an idea of how swollen they would be and which would tell them about their overall composition. And that’s exactly what they did in Figure 1! They even added two lines representing the rocky, half-water/half-rock “Planet Recipes”. Almost all planets fall well on one of the two lines. Planets that lie on the rock recipe line have a wider range of masses while half water / half rock have a mass of 2-3 earth masses minimum.
It should also be noted that the radii of a few planets are larger than expected relative to their mass and cannot be explained by any of the recipe. There are recipes that explain it, rocky planets with large gaseous atmospheres, or watery planets with puffy atmospheres due to the global warming effect (speaking of which, did you know the Astrobites have a streak of climate change?).
And the winner is…
So what can these recipes tell us about the planetary baking process? Planets must consist of a mixture of ice and rock that join together in a star’s protoplanetary disk. There are two main theories as to how this might happen. Both begin with the formation of a planet’s core, but then grow thanks to planetary accretion (a kilometer-sized substance collides and mixes) or gravel accretion (a centimeter-sized substance builds up over time). Since there are no planets among the full rock and half water/half rock recipes, this indicates gravel accumulation, as planetary accretion predicts the presence of some bodies of intermediate composition. This means that all stars’ “baking strategy” is the same, and the difference in the composition of their planets is where they are made In the protoplanetary disk, rocky planets form within the ice line (far enough distance from the host star beyond which the ice can exist), half of the water/half of the rocks left behind and possibly migrating inland later. For the multiple planetary systems in her sample, the rocky planets are always the inner planets and the others are watery worlds, so it seems appropriate!
While this result only used planets around M dwarfs, the authors speculate about more massive stars (such as our sun). They note that theoretical models predict similar results, and that simulations using models of planet formation and evolution support their findings. However, there are not enough well-constrained planet radii to perform the same analysis for F, G and K type stars. However, this is a huge step in understanding planet formation, and a very tasty result!
Astrobit Edited by Graham Doskoch
Featured image credit: Wikipedia and NASA/JPL-Caltech
About Mark Poppinchalk
I am a PhD candidate at CUNY/Hunter College at the American Museum of Natural History. I study the age of stars by measuring the speed of their rotation. I enjoy absolute Frisbee, baking bread and all kinds of games. My favorite color is sky blue and pink.