HR 8799: The Benchmark Directly-Imaged Planetary System

TL;DR

The HR 8799 system is a key benchmark for studying directly-imaged planets, providing insights into their atmospheres, orbits, and formation processes through multi-wavelength observations and long-term astrometry.

Contribution

This paper highlights HR 8799 as a benchmark system for understanding young superjovian planets, emphasizing its unique observational data and implications for planet formation theories.

Findings

HR 8799 planets have thicker clouds than similar brown dwarfs.

Evidence of non-equilibrium carbon chemistry linked to low surface gravity.

Orbits constrained to not be face-on, possibly in resonances.

Abstract

HR 8799 harbors arguably the first and best-studied directly-imaged planets. In this brief article, I describe how the HR 8799 planetary system is a benchmark system for studying the atmospheres, orbital properties, dynamical stability, and formation of young superjovian planets. Multi-wavelength photometry and spectroscopy show that HR 8799 bcde appear to have thicker clouds than do field brown dwarfs of similar effective temperatures and exhibit evidence for non-equilibrium carbon chemistry, features that are likely connected to the planets' low surface gravities. Over 17 years of astrometric data constrain the planets' orbits to not be face on but possibly in multiple orbital resonances. At orbital separations of 15--70 au and with masses of $\approx$ 5--7 $M_{\rm J}$, HR 8799 bcde probe the extremes of jovian planet formation by core accretion: medium-resolution spectroscopy may provide clues about these planets' formation conditions. Data from the next generation of 30 m-class telescopes should better constrain the planets' orbits, chemistry, gravity, and formation history.