My Thesis in 400 Words: Cheryl Wang

Cheryl Wang, an iREx Masters student, completed her M.Sc. at McGill University this summer. Here, she summarizes her Masters research project.

The process by which Earth-like planets form is still a topic of hot discussion among scientists. One possibility is that small “planetary embryos” come together by gathering up material from even smaller objects called planetesimals. Over time, these embryos grow to become about ten times the size of Earth. After a while, when the gas around them disappears, the embryos start falling onto each other due to gravitational interactions, resulting in collisions and the final formation of an Earth-sized planet. However, the exact conditions in the nebula and the timing of these embryo formations are still not completely clear.

Luckily, noble gases like neon, found deep inside Earth, give us hints about how planets form. Previous research has shown that the neon in Earth’s mantle matches the kind found in the early solar nebula, suggesting that proto-Earths formed before the solar nebula disappeared.

In the early stages of planet formation, gas in the nebula around the embryo keeps getting added to the growing embryos. The planet’s surface stays hot because of the heat generated during the embryo’s initial assembly. This allows volatile elements in the added gas to mix into a molten ocean on the planet. Over time, the nebula slowly spreads out, and the planet’s surface cools down, causing some of the trapped volatiles to escape from the now-solid surface. So, we can understand how gases moved between the atmosphere and the planet’s mantle by studying the atmosphere of protoplanetary embryos and using a principle called Henry’s law.

My master’s thesis project tries to figure out how much gas was in the protoplanetary disk based on the neon concentration in Earth’s mantle, considering that some gases escaped after solidification. Our research shows that the proto-Earth had to be at least 0.2 times the mass of Earth to have a molten ocean that could keep volatile gases. These embryos typically formed between 0.5 and 1 astronomical unit (AU) from the sun, about 20 million years after the solar system started, when the amount of gas in the disk has shrank by 100-1000 times. In short, we think Earth and other similar planets formed when there was still some nebular gas left, but it was just about to disappear.