Thousands of exoplanets have been discovered. Through the “transit technique,” astronomers have uncovered more and more of these new worlds within the orbital period of about only 100 days around Sun-like stars in the past few years. In particular, the transit survey conducted by NASA’s Kepler space telescope, with other follow-up observations, has revealed the so-called "radius valley," i.e., the low population for the planets of planetary radii ∼1.8 earth radii. This radius valley separates a large group of gaseous, hence puffy planets called sub-Neptunes from another large group of small, potentially rocky planets called super-Earths (see Figure 1). Atmospheric escape driven by extreme-ultraviolet photoevaporation has been one of the leading models to explain the presence of the radius valley. In this evolutionary scenario, the sub-Neptunes larger than the radius valley can retain the primordial hydrogen-rich atmosphere against atmospheric escape over billions of years, whereas the super-Earths smaller than the radius valley are originally sub-Neptunes but lose their primordial atmosphere later on and become the bare rocky cores. Pin-Gao Gu of Academia Sinica Institute of Astronomy and Astrophysics and Howard Chen of the Florida Institute of Technology recently predicted that these highly irradiated puffy planets near the radius valley are deuterium-enriched in their hydrogen-dominated atmospheres.
Why is studying the deuterium abundance important to astronomers? Deuterium (a.k.a. heavy hydrogen) is a sibling of hydrogen that has one proton and one neutron and is therefore twice the atomic mass of hydrogen. The deuterium-to-hydrogen (D/H) abundance ratio of Jupiter and Saturn is comparable to the protosolar value (i.e., 20 parts per million), consistent with giant-planet formation through massive gas accretion from the solar nebula. The D/H of Earth’s ocean is similar to that of carbonaceous chondrites, which may support the water delivery by asteroids from outer space. The D/H in Venus's atmosphere is much higher than that of Earth's ocean by about a factor of 100. Previous work has shown that the rapid evaporation of the putative early ocean via a runaway greenhouse effect can induce more loss of hydrogen to space than the heavier isotope D, leading to the high D/H ratio on Venus. Therefore, a similar phenomenon could also happen to these close-in sub-Neptunes that have been bombarded by more intense extreme-ultraviolet stellar photons than Venus. The resulting photoevaporating winds would lead to a more efficient escape of hydrogen than deuterium, thus enhancing the D/H value in their hydrogen-dominated atmospheres. “The D to H ratio provides crucial clues to planet formation and evolution," says Howard Chen.
Using computer simulations, Pin-Gao Gu and Howard Chen studied the abundance evolution of the hydrogen, deuterium, and helium in the atmospheres of close-in sub-Neptunes around Sun-like stars. The team found that a fraction of heavier D can bid farewell to the lighter hydrogen in the planetary winds over billions of years. This H-D fractionation is more prominent for the sub-Neptunes near the upper edge of the radius valley. This predictive outcome is akin to the puffy helium planets due to photoevaporating escape in the previous study by a team led by Isaac Malsky of the University of Michigan and Leslie Rogers of the University of Chicago. Assuming the initial D/H in the gas envelope of sub-Neptunes is protosolar, Figure 2 illustrates the D/H of the planets along the radius valley from this new study in comparison with those in Earth's ocean, as well as in the gas and icy planets in the solar system. “Prior to our work, the D/H has been studied for the interstellar medium, protoplanetary disks, chondrites, comets, and even Earth's ocean, but there has not yet been a rigorous attempt to quantify D/H in exoplanet atmospheres,” added Pin-Gao Gu, “For the first time, we constructed an evolutionary model to include exoplanets in the context of the D/H ratio for solar-system planets.” More hydrogen is leaving the puffy planets to dark outer space than deuterium – it sounds a bit sentimental for the life and death of the sibling compositions, reminding us of the song lyrics from the recent movie “Day Off”:
I'll leave, you take your time to go,
We're used to saying goodbye like this.
You turn to look at me, smiling sweetly.
If there's something to say, you find it unnecessary.
No matter how dark and distant the road, we're not alone.