Washington: Usually, planets form in star systems with comparatively high concentrations of elements heavier than hydrogen and helium, a new study has suggested.
Such heavier elements are necessary to form the dust grains and planetesimals that build planetary cores, according to the study, which was carried out by researchers Jarrett Johnson and Hui Li of Los Alamos National Laboratory in New Mexico.
Additionally, evidence suggests that the disks of dust that surround young stars don’t survive as long when the stars have lower concentrations of heavy elements, or lower “metallicities” in astronomers’ jargon.
The most likely reason for this shorter lifespan is that light from the star causes clouds of dust to evaporate.
“Our calculation is an estimate of the minimum amount of heavy elements that must be present in circumstellar disks before planets can form,” Johnson said.
“Because these heavy elements must be produced by the first stars in the universe, the first planets could only form around later generations of stars.”
Understanding how the first planets formed provides crucial information about the early universe. Additionally, a better understanding of early planetary formation impacts many facets of astronomy, including the search for life elsewhere, researchers said.
According to Johnson and Li, a successful theory of planet formation should make predictions about the properties of the earliest planets and their host stars.
Such a theory could be tested by studying very old planetary systems in our galaxy. The enrichment of gas with metals from supernovas is thought to affect not only planetary formation, but the formation of low-mass stars like our sun as well.
“A planet as massive and dense as the Earth could only form once stars and supernovae had enriched the gas with an abundance of heavy elements that is at least 10 percent that in the sun,” Johnson said.
This suggests that many generations of stars had to form and evolve before habitable planets could form.”
One important consideration for planetary formation is the dispersal rate of the circumstellar disk of gas and dust around a host star.
Two of the more prominent mechanisms for dispersing a planetary disk are giant planet formation and photoevaporation by the host star.
Photoevaporation appears to be the more dominant process. Observations show that low-metallicity disks have shorter lifetimes, which is bolstered by data showing higher-metallicity disks are better “shielded” from evaporation by a host star’s radiation.
Johnson and Li further assert that disks with higher metallicity tend to form a greater number of high-mass giant planets.
In order to obtain estimates of the critical metallicity necessary for planet formation, Johnson and Li compared the lifetime of the disk and the length of time required for dust grains in the disk to settle.
Basically, for a star system to form planets, the time required for dust grains to settle cannot exceed the disk’s lifetime.
Since the settling time for dust grains depends on the density and temperature of the disk, which are related to the distance from the host star, the critical metallicity is also a function of distance from the host star.
The team noted several assumptions made in their comparisons with the data. The first assumption is that surface metallicity of the host star is the same as that of the protostellar disk from which it and its planets formed.
Second, the team assumed circular planetary orbits. When orbits are highly eccentric, comparing the data to the theoretical predictions is more difficult. Lastly, the team assumes planets have not migrated inward toward their star from their initial place of birth in the disk.
Johnson and Li found that the formation of planetesimals can only take place once a minimum metallicity is reached in a protostellar disk.
Since the earliest stars that formed in the universe (Population III stars) do not have the required metallicity to host planets, it is believed that the supernova explosions from such stars helped enrich subsequent (Population II) stars, some of which may still be in existence and could host planets.
The study will appear in the Astrophysical Journal.