The discovery of gas disks around main-sequence stars well beyond the time when planets have already formed could be good news for terrestrial exoplanets, as a new study shows that gas accretion from the disk, can replace the rocky worlds hydrogen-filled primordial atmosphere with a new second atmosphere rich in carbon and oxygen.
In the last few years, scientists have discovered that planetesimal belts similar to our Kuiper belt – disks of material filled with volatile-rich planetesimals – could be a staple feature around many stars; at present more than 25 percent of stars have planetesimal belts massive enough to be tracked down by our telescopes through the infrared light given off by these systems, although the figure is expected to be much higher when our detection limits improve.
These belts, which not only seem to be ubiquitous, are older than 10 Million years (Myr) and can last for hundreds of millions of years.
Belts older than 10 Myr are already expected to be populated with growing or fully established planets. For example, based on cosmochemical evidence, Mars is thought to have formed very early (less than 10 Myr) and while the Earth took slightly longer (10–30 Myr), most of its mass was acquired within a 10 Myr period.
Accompanying the belts are large disks of gas that are thought to form when the left-over planet building material smash into one another releasing carbon and oxygen as they bump their way around the belt.
These late gas disks, as they have become known, are therefore very different from the younger (< 10 Myr) and much more massive and hydrogen rich protoplanetary disks which provided the blanket of material to kick-start the birth of any budding planets.
These differences, also have another interesting consequence. Using computer simulations to model planetary atmospheres in late gas disks, a team of researchers in Paris found that even when these disks were diluted or tenuous, an atmospheric mass equivalent to that of Earth’s, could be readily accreted on to a terrestrial exoplanet milling around in the disk.
In disks that were slightly more massive, enough carbon dioxide (CO) could be accreted to give an exoplanet sub-Neptune-like atmospheric pressures.
“We show the effect of gas accretion on a planet’s pressure and bulk density over time and confirm that a variety of pressures from Earth-like (1 bar) to sub-Neptune-like (>104 bar) can be reached on low-mass planets formed in these disks,” write the team, headed by Quentin Kral at LESIA, Observatoire de Paris, in their research paper submitted to Nature today.
Furthermore, a late gas accretion scenario is a more favourable and efficient method for accreting volatiles on a terrestrial planet than if it was bombarded my comets and meteorites, say the team - a situation that many think was responsible for delivering large amounts of water and other volatiles to our planet around 3.8 billion years ago.
“Even if a late heavy bombardment-like event happens several hundreds of millions of years after the gas disk dissipated, the atmosphere would still be dominated by volatiles accreted by late gas accretion,” explain Kral and colleagues.
This can be tested by looking at an evolved planets C/O ratio when future observatories such as James Webb Space Telescope (JWST), the Extremely Large Telescope (ELT), and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) have the ability to look at spectra for many more terrestrial-like exoplanets.
It is also good news for planets that are either close-in or at lengthy distances from their host stars or for planets that did not have much of an atmosphere to begin with, as the late-accretion would replace or fill in the bulk of these atmospheres, even for cases where the disk is not very massive, say the team.
For sub-Neptunes or more massive planets that already have substantial hydrogen-rich atmospheres, the late disk accretion is likely to mix up the gases and increase their metallicities (in astronomy, any element heavier than helium is considered a metal), by as much as 1000 times the value of our Sun’s metallicity, or less for more massive Jupiter-like planets.
Variations such as these have already been observed in a number of exoplanets, say the team such as GJ 3470 b. With a mass of just under 14 Earth-masses and a radius approximately 4.3 times that of Earth's, GJ 3470 b has a near-solar metallicity.
Other Neptune-sized exoplanets like K2-18 b, HAT-P-26 b, and HAT-P-11 b, which have masses, 8.9, 22 and 26.7 times that of Earth, respectively, all have super-solar metallicities, meaning that the abundance of elements heavier than hydrogen and helium is a lot greater than that found in our Sun; depending on which study you follow, the total amount of metals by mass in the Sun, is about 1.3 - 1.8 per cent so the rest is made up of hydrogen or helium.
These metallicity effects can occur early in the planet’s history and could be an indicator of late-disk gas accretion.
It is shown from other studies that the gas accretion rate doesn’t depend too much on the gas density from which it accretes, but rather on the planet’s ability to cool or radiate away its energy (although it is limited by the quantity of gas available).
The more a planet can cool, the more it contracts, leaving an area around the planet known as a Hill sphere* (see below) to get refilled rapidly. So the further a planet is away from its host star, the quicker it should be able to cool down.
In their study, Kral and colleagues say their new accretion model is expected to be very efficient, and that the outermost planets should accumulate more of the disk material before it has time to spread further in towards the host star.
Because CO atmospheres have a much higher mean molecular weight compared with hydrogen-dominated atmospheres, this could account for why some planets in a system differ so much than others, such as those in the now famous TRAPPIST-1 system.
Hubble observations two years ago, revealed that at least three of the exoplanets (d, e, and f) do not seem to contain puffy, hydrogen-rich atmospheres. At the time, the fourth planet's (g) atmosphere was unknown, but a layer of volatiles were identified in planets b, d, f, g, and h suggesting either a water shell, an ice shell, or a thick atmosphere.
Finally, say the team, as a large fraction of the disk gas is accreted rather than passed on further in, the disks will often be very depleted inwards of a planet, a feature that could help astronomers indirectly infer the existence of a low-mass planet at a few astronomical units or more away from their host star.
These results could be narrowed down further using the high spatial resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) to help pinpoint the planet location.
*A Hill sphere is the radius of a sphere within which smaller bodies would tend to orbit the body. Outside the radius, the body would be drawn to orbit around the next larger body which the initial hosting body is orbiting.