Fingerprints of giant planets in the composition of solar twins

Abstract

The Sun shows a ∼ 10 % depletion in refractory elements relative to nearby solar twins. It has been suggested that this depletion is a signpost of planet formation. The exoplanet statistics are now good enough to show that the origin of this depletion does not arise from the sequestration of refractory material inside the planets themselves. This conclusion arises because most sun-like stars host close-in planetary systems that are on average more massive than the Sun's. Using evolutionary models for the protoplanetary discs that surrounded the young Sun and solar twins we demonstrate that the origin of the depletion likely arises due to the trapping of dust exterior to the orbit of a forming giant planet. In this scenario a forming giant planet opens a gap in the gas disc, creating a pressure trap. If the planet forms early enough, while the disc is still massive, the planet can trap 100 M ⊕ of dust exterior to its orbit, preventing the dust from accreting onto the star in contrast to the gas. Forming giant planets can create refractory depletions of ∼ 5 − 15%, with the larger values occurring for initial conditions that favour giant planet formation (e.g. more massive discs, that live longer). The incidence of solar-twins that show refractory depletion matches both the occurrence of giant planets discovered in exoplanet surveys and "transition" discs that show similar depletion patterns in the material that is accreting onto the star.

The chemical make-up of the Sun: A 2020 vision

“…Recently, a more promising scenario was proposed by Booth & Owen (2020), which involves the early formation of giant planets trapping >100 M ⊕ of refractory-rich dust external to their orbits while volatile-rich gas continues to accrete onto the proto-star. They found that such gas-dust separation could imprint an abundance pattern similar to that observed when the Sun is deficient in refractory elements by ≈0.08 dex (20%).…”

A dusty origin for the correlation between protoplanetary disc accretion rates and dust masses

“…where f grow is an extra factor used by e.g. Ormel et al (2017); Booth & Owen (2020) which is an (inverse) measure of the fraction of dust grain collisions that lead to growth. Growth stops once the dust size reaches the lower of the fragmentation-limited or drift-limited regime (Birnstiel et al 2012):…”

“…Gundlach & Blum (2015) Initial/Minimum size (a 0 ) 0.1 µm -Initial dust-to-gas ratio ( ) 0.01 Bohlin et al (1978) Growth factor ( f grow ) 1 10Birnstiel et al 2012; (Booth & Owen 2020) X-ray Luminosity (L X ) 0 erg s −1 (5 × 10 28 , 10 30 , 10 31 erg s −1 ) (Preibisch et al 2005;Güdel et al 2007…”

“…This parameter has been used in models of pebble accretion (Ormel et al 2017). Moreover Booth & Owen (2020) explored whether f grow > 1 could rectify the problem that dust evolution models frequently deplete dust on a faster timescale than observations suggest, specifically that for ] f grow = 1 dust depletes before gap opening by giant planets could create a transition disc. For f grow = 10, they found that dust was removed more slowly and this could help explain the depletion of refractory elements in the Sun.…”

On the secular evolution of the ratio between gas and dust radii in protoplanetary discs

“…where Ω is the Keplerian angular frequency and the growing factor 𝑓 grow is an extra factor (considered to be equal to 1 in the "standard" two population model and in this work) used e.g in Booth & Owen 2020 to measure the fraction of collisions that lead to growth: 𝑓 grow = 1 means that all the collisions result in dust growth while 𝑓 grow = 100 means that only 1% of the collisions result in growth, leading to longer lifetimes for dust in discs due to a less efficient radial drift (for an extended discussion see Booth & Owen 2020or Sellek et al 2020).…”