Images of a fourth planet orbiting HR 8799

TL;DR

The discovery of a fourth planet orbiting HR 8799 challenges existing planet formation theories, as current models cannot explain the in situ formation of all four giant planets at wide separations.

Contribution

This paper reports the detection of a fourth planet in the HR 8799 system, highlighting the system's complexity and its implications for planet formation models.

Findings

Identification of a fourth planet interior to the other three.

The HR 8799 system's configuration challenges current planet formation theories.

Presence of debris belts associated with the system.

Abstract

High-contrast near-infrared imaging of the nearby star HR 8799 has shown three giant planets. Such images were possible due to the wide orbits (> 25 AU) and youth (< 100 Myr) of the imaged planets, which are still hot and bright as they radiate away gravitational energy acquired during their formation. A major area of contention in the extrasolar planet community is whether outer planets (> 10 AU) more massive than Jupiter form via one-step gravitational instabilities or, rather, via a two-step process involving accretion of a core followed by accumulation of a massive outer envelope composed primarily of hydrogen and helium. Here we report the presence of a fourth planet, interior to and about the same mass as the other three. The system, with this additional planet, represents a challenge for current planet formation models as none of them can explain the in situ formation of all four planets. With its four young giant planets and known cold/warm debris belts, the HR 8799 planetary system is a unique laboratory to study the formation and evolution of giant planets at wide > 10 AU separations.