The Solar Twin Planet Search

Abstract

Context. More than two thousand exoplanets have been discovered to date. Of these, only a small fraction have been detected around solar twins, which are key stars because we can obtain accurate elemental abundances especially for them, which is crucial for studying the planet-star chemical connection with the highest precision. Aims. We aim to use solar twins to characterise the relationship between planet architecture and stellar chemical composition. Methods. We obtained high-precision (1 m s −1 ) radial velocities with the HARPS spectrograph on the ESO 3.6 m telescope at La Silla Observatory and determined precise stellar elemental abundances (∼0.01 dex) using spectra obtained with the MIKE spectrograph on the Magellan 6.5 m telescope. Results. Our data indicate the presence of a planet with a minimum mass of 26 ± 4 Earth masses around the solar twin HIP 68468. The planet is more massive than Neptune (17 Earth masses), but unlike the distant Neptune in our solar system (30 AU), HIP 68468c is close-in, with a semi-major axis of 0.66 AU, similar to that of Venus. The data also suggest the presence of a super-Earth with a minimum mass of 2.9 ± 0.8 Earth masses at 0.03 AU; if the planet is confirmed, it will be the fifth least massive radial velocity planet candidate discovery to date and the first super-Earth around a solar twin. Both isochrones (5.9 ± 0.4 Gyr) and the abundance ratio [Y/Mg] (6.4 ± 0.8 Gyr) indicate an age of about 6 billion years. The star is enhanced in refractory elements when compared to the Sun, and the refractory enrichment is even stronger after corrections for Galactic chemical evolution. We determined a nonlocal thermodynamic equilibrium Li abundance of 1.52 ± 0.03 dex, which is four times higher than what would be expected for the age of HIP 68468. The older age is also supported by the low log (R HK ) (-5.05) and low jitter (<1 m s −1 ). Engulfment of a rocky planet of 6 Earth masses can explain the enhancement in both lithium and the refractory elements. Conclusions. The super-Neptune planet candidate is too massive for in situ formation, and therefore its current location is most likely the result of planet migration that could also have driven other planets towards its host star, enhancing thus the abundance of lithium and refractory elements in HIP 68468. The intriguing evidence of planet accretion warrants further observations to verify the existence of the planets that are indicated by our data and to better constrain the nature of the planetary system around this unique star.

Super-Earth ingestion can explain the anomalously high metal abundances of M67 Y2235

“…However, the third turn-off star, Y2235, has enhanced elemental abundances compared to the other two, as shown in Figure 1. The differences show no particular trend with atomic number or sublimation temperature (unlike between some pairs of stars; see Meléndez et al 2009Meléndez et al , 2017Ramírez et al 2015) and, to within the observational uncertainties, are consistent with a constant offset of 0.128 dex in each species. The presence of a systematic offset is significant to about six sigma.…”

“…This latter process is particularly effective if the accretion takes place once the host star has reached the main sequence, as its convective envelope is then small and the material of the planet can provide a large abundance enrichment if it mixes into a relatively small fraction of the star. Often the required quantities of material are estimated to be a few Earth masses (e.g., Meléndez et al 2017;Liu et al 2018). This places potential ingested planets in the class of super-Earths, a class which has been shown to be extremely common, with transit and radial-velocity surveys giving oc-5000 5500 6000 6500 currence rates of 30 − 70% (Mayor et al 2011;Fressin et al 2013;Zhu et al 2018;Zink et al 2019).…”

Signatures of rocky planet engulfment in HAT-P-4

“…It is also supported by the slightly higher mass of the A star (corresponding to a lower convective mass and lower mixing of the accreted material) and by the non-detection of additional inner planets. A very similar scenario was recently proposed for the solar-twin star HIP 68468 (Meléndez et al 2017), where the authors suggested that the possible inward migration of a close-in giant planet likely produces the engulfment of the inner planetary material by the star. This would explain the high Li content (0.6 dex greater than expected for its age), a refractory enhancement compared to the Sun, and a T c trend.…”

The Chemical Signatures of Planetary Engulfment Events in Binary Systems

“…In fact, when studying samples of twin stars, most of the systematic uncertainties that plague chemical abundance analyses are so similar among stars in the same binary pair that a strict differential line-by-line analysis cancels them out. Also NLTE effects have been found negligible for all the elements considered by our analysis of solar twin stars (Meléndez et al 2012;Melendez et al 2016;Nissen 2015;Spina et al 2016), and so no NLTE corrections were required. This leaves the observational noise as the main source of error.…”