A un planeta al rojo vivo le crecen dos colas exprés
El planeta WASP-121b está tan cerca de su estrella que el calor hace que su atmósfera se evapore y se escape, como las colas de un cometa. Con el telescopio James Webb, los astrónomos observaron por primera vez este proceso de forma continua. Resultó que el helio de la atmósfera se estira en dos plumas gigantes que abarcan el 60% de la órbita, es decir, cientos de millones de kilómetros. Una cola, empujada por la radiación estelar, adelanta al planeta y cae sobre la estrella. La otra se extiende detrás y se aleja. Así, el planeta se convierte en algo parecido a un cometa, perdiendo su envoltura gaseosa. Antes, estas colas dobles no se habían podido ver en su totalidad. Esto ayuda a comprender qué tan rápido mueren los mundos gaseosos alrededor de otros soles. Las observaciones se realizaron en el brillo especial del helio, que permite al análisis de la luz ver incluso las capas más tenues de la atmósfera, como si miráramos en las profundidades del clima espacial.
Resumen original
tal como lo escribieron los autores, en inglés arXiv:2510.09809
Atmospheric escape of planets on short orbital periods, driven by the host star's irradiation, influences their evolution, composition, and atmospheric dynamics. Our main avenue to probe atmospheric escape is through the near-infrared metastable helium triplet, which has enabled mass loss rate measurements for tens of exoplanets. Among them, only a few studies show evidence for out-of-transit absorption, supporting the presence of a hydrodynamic outflow. However, none of these observations precisely identified the physical extent of the outflow, either due to non-continuous or short-duration observations. This limits our measurements of accurate mass loss rates. Here we present the first continuous, full-orbit helium phase curve monitoring of an exoplanet, the ultra-hot Jupiter WASP-121b, obtained with JWST/NIRISS. It reveals helium absorption for nearly 60% of the orbit at >3sigma significance. Our results show that WASP-121b sustains a strong outflow, separating into two tails trailing and leading the planet. The persistent absorption from these tails, together with their measured radial velocity shifts, suggests that they remain in a collisional fluid regime at large distances from the planet and display very different dynamics. The leading trail has a higher density and moves toward the star, whereas the trailing trail is being pushed away from the star, with the latter being blue-shifted due to stellar irradiation pressure. While qualitatively agreeing with theoretical expectations, the observed structure of helium is not self-consistently reproducible by current models, limiting constraints on the mass loss rate. Furthermore, we show that while ground-based observations of the helium triplet are essential to measure the outflow dynamics precisely, they ideally should be combined with continuous JWST phase curves to constrain the absolute level of helium absorption.
El brillo especial del helio permite asomarse a las capas superiores e hinchadas de las atmósferas de exoplanetas, allí donde la luz ordinaria no llega.