Planet Formation and the Radius Valley


We’re pretty familiar with our own solar system. In addition to our planet Earth, It contains the well-known planets Venus, Mercury, Mars, Jupiter, Saturn, Uranus, and Neptune. It also contains some dwarf planets, including Pluto, a plethora of asteroids, and, of course, our Sun. But we are not the only solar system in our galaxy. In fact, there are many, many more, and we are only just now beginning to learn more about them. Planets outside of our solar system are called “exoplanets.” They each have their own star, like our Sun, that they orbit. The Kepler mission, which launched in March of 2009, has helped us learn a lot about these exoplanets and what other planetary systems in our galaxy look like. Thanks to the Kepler mission, we have discovered thousands of new planets and collected data about them. In analyzing the data from the Kepler mission, scientists have discovered a very large amount of planets that are unlike any within our own solar system. Their sizes are between those of Earth and Neptune, but they orbit extremely close to their stars. In our solar system, Mercury orbits closest to the Sun, but these exoplanets, known as “Super-Earth”/”Sub-Neptune”s orbit even closer to their stars than Mercury does. Even though planets like these aren’t found anywhere within our own solar system, they actually are the most common type of planet in our galaxy, and probably the universe, known to date. These planets have another interesting feature: their radii. They have a bimodal distribution in size, meaning that if you were to plot the sizes of all these Super-Earth/Sub-Neptunes on a histogram, you would come up with two peaks and a noticeable valley in the middle. This valley lands at around 1.7 Earth radii. The radius valley has intrigued scientists, and several new studies have attempted to explain its cause. If these planets are so common, why are there hardly any around this size? Discoveries made by recent studies have revealed potential causes of this radius valley. These studies suggest that both types of these exoplanets, the Super-Earths and the Sub-Neptunes, formed as one population. This means that they were originally the same type of planet, but then some of them got progressively smaller in size. How does this happen? Well, first you need to know about the process of planet formation and the importance of a planet’s hydrogen/helium envelope. Planets form in protostellar disks. These disks are the birthplaces of planets, and contain all the raw material, dust and gas, necessary for a planet’s formation. As planets form from these disks, they start out as dust grains that grow into larger aggregates. Think of it like dust bunnies in your bedroom, starting out as one piece of dust and getting bigger and bigger in time. From there, these planets grow into asteroid-sized and Mars-sized bodies. This process is called “accretion.” As they get bigger and bigger during accretion, the planets’ gravity attracts a large amount of hydrogen and helium to them, which forms a hydrogen/helium envelope around them. These envelopes have a huge effect on the planet’s size. For example, even if a planet has just 1% of hydrogen/helium gas in its atmosphere, its radius essentially doubles due to the low density of the gas. Recent studies propose that the exoplanets we’ve been discussing, the Super-Earths and the Sub-Neptunes, started off with hydrogen/helium envelopes. This means that they were all initially very similar sizes, and probably would have landed on the higher end of the histogram that we plotted earlier. However, during the planetary formation and evolution process, some of the planets lost their hydrogen envelopes and hence dramatically decreased in size. Work by the Planet Formation Group at UCLA proposes that this atmospheric mass-loss is driven by the thermal energy, or heat, that is stored in a planet’s core during planet formation. The source of this thermal energy is gravitational binding energy that is converted into heat as the planets grow by accreting dust and gas. This causes planetary cores to be extremely hot. This heat becomes especially important for planets that have accreted a hydrogen/helium envelope, since these envelopes act as thermal blankets engulfing the core. As thermal energy is gradually released, it can cause the hydrogen envelope to unbind and strip the hydrogen/atmosphere that sits on top of the core. The stripping of the hydrogen/helium envelope causes the planets’ radii to become drastically smaller. These planets, with stripped hydrogen/helium envelopes, are the Super-Earths, the planets on the lower end of the radius valley. However, not all planets are stripped. Planets can save their hydrogen envelopes in two ways. First, some planets don’t have enough thermal energy from planet formation stored in their cores to unbind their hydrogen envelopes. Second, some planets’ envelopes can cool faster than the planet loses mass. The mass-loss timescale and the cooling timescale are in competition with each other. If the cooling timescale is short enough, meaning that the planet cools faster, then the hydrogen/helium envelope will shrink. Even the slightest shrinking of the envelope will cut off most of the mass loss, so the process essentially stops. In both these cases, the hydrogen/helium envelope will not be stripped. Instead, it remains on top of the planet’s core, causing the radius of the planet to be significantly larger. These planets are the Sub-Neptunes, the planets on the upper end of the radius valley. This disparity in size leads to the bimodal distribution, or “radius valley,” that we can see in the histogram. So, what do these findings tell us? Well, for one thing, they show that the radius valley can be explained by the process of planet formation itself. Additionally, using the core-powered mass-loss model, we can make a series of observational predictions. For one, we can predict that the radius valley should move to larger planet sizes for planets that formed around more massive stars. This is because higher-mass stars have larger luminosities, meaning they shine brighter. The higher luminosity causes any planet orbiting the star to lose its hydrogen/ helium envelope faster than it would if it was orbiting a lower-mass star. As a result, even more massive cores can be stripped of their envelopes. This model also tells us that, since the timescale for envelope loss can range from several hundred millions of years to ten billion years, some planets should be undergoing atmospheric loss even today. Hence, the abundance of Super-Earths relative to Sub-Neptunes increases with time. Research like this helps us understand how and why planets form the way they do. It gives us a better idea of how our own solar system came to be, and what we can expect from other solar systems. It moves us a little closer to understanding our galaxy and our place within it. Though there is still much we don’t know about the universe, by asking questions and finding ways to answer them, we can learn a little bit more every day.

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