Metal-rich environment crucial for light giant planets, but not for heavy giant ones: Study

A metal-rich environment of host stars is vital for the formation of Jupiter-type light, but it is not necessary for the long orbit of heavy giant planets, research on exoplanets has found.

Planets called exoplanets orbit stars similar to the Sun, forming their own stellar system.

Researchers from the Indian Institute of Astrophysics (IIA), under the Department of Science and Technology (DST), and the Tata Institute of Fundamental Research investigated the host star properties for the directly imaged exoplanets to understand various formation scenarios for these wide orbit celestial objects.

This study, which explored connections between the planet and host star properties, can help in understanding how planets form and evolve at large orbital distances.

With more than 4,300 planets discovered to date, it has become essential to characterise the exoplanets in terms of their various properties, the DST said.

Correlation between star and planet properties can provide vital clues about the possible formation and evolution scenarios.

Stars are largely made of hydrogen and helium with a small fraction of other elements.

In astronomical lingo, elements heavier than hydrogen and helium are collectively called metals.

Metal content is an important parameter of the star, and there is a consensus that planets (small or large) are more likely to occur around metal-rich stars.

Although the stellar metallicity (property of being metallic) and planet occurrence rates for close-by systems have been investigated by many research groups, properties of stellar hosts of exoplanets, especially those located at large orbital distances, are not very well studied.

While scientists studying the short-orbit exoplanets had earlier found that a host star with a metal-rich environment provides the favourable condition for the formation of Jupiter-type low-mass giant planets, the new study published in the Astronomical Journal suggests that this is not necessarily the case for the long orbit high-mass giant planets discovered by the direct imaging technique.

This finding seems consistent with an existing model called the core accretion model of planet formation, the DST added.

For planet mass greater than the mass of Jupiter, the large scatter in metallicity distribution suggests that metallicity might not play a significant role in the formation of these celestial objects.

This means there is no single dominating mechanism for planet formation at wider orbits. Planets in remote orbits could form either by core accretion process or gravitational instability, the DST said.