Direct detection and characterization of M-dwarf planets using light echoes

TL;DR

This paper proposes a novel method to detect and characterize M-dwarf exoplanets by utilizing light echoes generated during stellar flares, expanding detection capabilities without relying on transits or coronagraphs.

Contribution

It introduces a new approach leveraging stellar flares and light travel time effects to enhance exoplanet detection and characterization around active M dwarf stars.

Findings

Light echoes can be distinguished from star signals by time delay, Doppler shift, and polarization.

Scattered light reveals planetary albedo spectra and polarization signatures.

Detection feasibility is promising under favorable conditions.

Abstract

Exoplanets orbiting M dwarf stars are a prime target in the search for life in the Universe. M dwarf stars are active, with powerful flares that could adversely impact prospects for life, though there are counter-arguments. Here, we turn flaring to advantage and describe ways in which it can be used to enhance the detectability of planets, in the absence of transits or a coronagraph, significantly expanding the accessible discovery and characterization space. Flares produce brief bursts of intense luminosity, after which the star dims. Due to the light travel time between the star and planet, the planet receives the high intensity pulse, which it re-emits through scattering (a light echo) or intrinsic emission when the star is much fainter, thereby increasing the planet's detectability. The planet's light echo emission can potentially be discriminated from that of the host star by means of a time delay, Doppler shift, spatial shift, and polarization, each of which can improve the contrast of the planet to the star. Scattered light can reveal the albedo spectrum of the planet to within a size scale factor, and is likely to be polarized. Intrinsic emission mechanisms include fluorescent pumping of multiple molecular hydrogen and neutral oxygen lines by intense LyAlpha and LyBeta flare emission, recombination radiation of ionized and photodissociated species, and atmospheric processes such as terrestrial upper atmosphere airglow and near infrared hydroxyl emission. We discuss the feasibility of detecting light echoes and find that under favorable circumstances, echo detection is possible.