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An Extended Escaping Tail of Hydrogen and Helium in a Potential Super-Earth Progenitor

Using JWST/NIRISS SOSS observations, researchers detected an extended, escaping tail of hydrogen and helium from the 23-million-year-old potential super-Earth progenitor V1298 Tau c, providing direct empirical evidence of the rapid atmospheric mass loss that shapes the evolution of young planets.

Original authors: Saugata Barat, Shreyas Vissapragada, Matthew Murphy, Darryl Seligman, Girish Duvvuri, Adina Feinstein, Luis Welbanks, William Misener, Sara Seager, Andrew Vanderburg, Rafael Luque, Kevin France, Benja

Published 2026-09-29

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Original authors: Saugata Barat, Shreyas Vissapragada, Matthew Murphy, Darryl Seligman, Girish Duvvuri, Adina Feinstein, Luis Welbanks, William Misener, Sara Seager, Andrew Vanderburg, Rafael Luque, Kevin France, Benjamin Rackham, Andrew Mann, Jean-Michel Désert, James Owen, Eva-Maria Ahrer, Sagnick Mukherjee, Michael Radica, Matthew Nixon, Keighley Rockliffe, John Livingston, Hinna Shivkumar, Sydney Petz

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Most of the planets in our galaxy are not the rocky worlds like Earth or the massive gas giants like Jupiter. Instead, they fall into a middle category: planets slightly larger than Earth, often called super-Earths, or those with thick, puffy atmospheres known as sub-Neptunes. For years, astronomers have wondered how these two groups came to be so different. One leading idea suggests that many of these planets were born as "gas dwarfs"—rocky cores wrapped in thick blankets of hydrogen and helium gas. Over time, the intense radiation from their host stars might have stripped away these gaseous coats, leaving behind the bare rocky cores we see today. However, this process is difficult to study because it happens over millions of years, and we usually only see the planets after the stripping is finished. To truly understand how these atmospheres vanish, scientists need to catch a planet in the act of losing its gas, a moment that requires observing a very young world while it is still in the early stages of its life.

A team of astronomers has now done exactly that, using the James Webb Space Telescope to observe a young planet named V1298 Tau c. This world is only 23 million years old, a mere infant in cosmic terms, and it orbits a star that is also very young. The planet is currently about five times wider than Earth and has a mass roughly five times that of our home planet. By pointing the telescope at this system, the researchers were able to watch the planet pass in front of its star and analyze the starlight filtering through the planet's atmosphere. What they found was a dramatic scene of atmospheric loss. Instead of a neat, spherical shell of gas surrounding the planet, they detected a massive, elongated tail of escaping hydrogen and helium stretching far behind the planet, trailing like the wake of a boat moving through water.

The researchers used a special instrument on the telescope that can see light in infrared wavelengths, which allowed them to detect specific chemical fingerprints in the escaping gas. They looked for three different signals simultaneously: a specific type of helium atom that is excited by starlight, and two different forms of hydrogen that glow when energized. All three signals showed a deep dip in starlight as the planet crossed the star, but the timing of these dips told a surprising story. The signals for helium and one type of hydrogen began to drop at the same time the solid body of the planet started to block the star, but they did not return to normal levels until hours after the planet had finished crossing. This delay proved that the gas was not just hugging the planet; it was streaming away in a long, trailing tail that extends tens of times the width of the planet itself.

This observation provides the first simultaneous proof of such an extended tail using multiple types of gas. The presence of these specific hydrogen signals so far from the planet's surface indicates that the gas is flowing out with incredible speed and density. The researchers calculated that the planet is losing mass at a rate of over 300 trillion grams every second. To put this in perspective, this is a rate so high that if it continued, the planet would lose its entire gaseous envelope in just the next ten million years, transforming from a puffy gas dwarf into a bare, rocky super-Earth. The data also suggests that the escaping gas is not a perfect mix of hydrogen and helium; it appears to be losing helium at a different rate than hydrogen, resulting in a mixture that is poorer in helium than the sun.

The study also helps settle a long-standing debate about how these atmospheres are lost. There are two main theories: one suggests that the star's high-energy radiation blows the atmosphere away like a wind, while the other suggests that the heat from the planet's own interior drives the gas off. The observations of V1298 Tau c are consistent with both ideas, but the sheer intensity of the mass loss leans slightly toward the idea that the planet's internal heat is playing a major role in driving this extreme escape. By catching this young planet in the act, the researchers have provided a crucial snapshot of planetary evolution. They have shown that the transition from a gas-rich world to a rocky one is not a slow, quiet fade, but a violent, rapid event where the planet sheds its primordial atmosphere in a spectacular, comet-like display. This single observation serves as a benchmark for understanding how the most common planets in the galaxy lose their atmospheres and what they eventually become.

Technical Summary: An Extended Escaping Tail of Hydrogen and Helium in a Potential Super-Earth Progenitor

Problem Statement
The origins of the galaxy's most common planets—super-Earths (1–1.5 $R_\oplus$ ) and sub-Neptunes (2–4 $R_\oplus$ )—are often attributed to the "gas-dwarf" hypothesis, where these bodies form with rocky cores enveloped in primordial hydrogen/helium (H/He) atmospheres that are subsequently eroded. While theoretical mechanisms such as photoevaporation, core-powered mass loss, and boil-off have been proposed to drive this erosion, empirical constraints on these early evolutionary processes (specifically within the first 100 million years) remain limited. A significant challenge in characterizing atmospheric escape in young systems is the degeneracy of atmospheric model parameters (e.g., temperature, mass-loss rate, and H/He abundance) and the difficulty of distinguishing planetary signals from intrinsic stellar variability. Furthermore, ground-based observations often lack the temporal baselines required to fully capture extended, trailing tails of escaping gas, which can last significantly longer than the broadband transit duration.

Methodology
The authors present observations of the primary transit of V1298 Tau c, a 23 Myr-old potential super-Earth progenitor ( $5.1 R_\oplus$ , $4.7 M_\oplus$ ), using the James Webb Space Telescope (JWST) NIRISS/SOSS instrument (R $\sim$ 700) as part of the KRONOS large program (PID 5959). The observation covered a total exposure of $\sim$ 9.6 hours, including extensive pre- and post-transit baselines to capture the full extent of any extended exosphere.

Key methodological steps included:

Data Reduction: Utilizing the exotedrf pipeline to process time-series spectroscopy from 0.85–2.83 $\mu$ m, correcting for 1/f noise and instrumental artifacts.
Multi-Tracer Detection: Simultaneous analysis of three independent tracers of atmospheric escape: metastable helium (He*) at 1.083 $\mu$ m, hydrogen Paschen- $\alpha$ (Pa- $\alpha$ ) at 1.875 $\mu$ m, and hydrogen Paschen- $\beta$ (Pa- $\beta$ ) at 1.282 $\mu$ m.
Light Curve Modeling: Employing the harmonica code to model the transit light curves. The authors modeled the outflow geometry as an elongated ellipse (trailing tail) rather than a spherically symmetric cloud, fitting parameters for trailing tail length ( $l_1$ ), leading tail length ( $l_2$ ), and height ( $h$ ).
Atmospheric Modeling: Using the sunbather code to jointly model the absorption signals. This involved a grid of isothermal Parker wind models coupled with the non-local thermodynamic equilibrium (NLTE) code Cloudy to calculate excited state abundances. The grid varied mass-loss rates ( $\dot{M}$ ), initial isothermal temperatures ( $T_0$ ), and He/H abundance ratios.
Validation: The study included rigorous testing of normalization methods, temporal windows for in-transit integration, and the impact of stellar spot crossings and the transit light source effect (TLSE).

Key Results

Detection of Extended Outflow: The study detected excess planetary absorption in He*, Pa- $\alpha$ , and Pa- $\beta$ simultaneously. The absorption in He* and Pa- $\alpha$ persisted for $\sim$ 2 and 1.5 hours, respectively, after the egress of the broadband white light curve. This provides strong evidence for a trailing, comet-like tail of outflowing gas extending tens of planetary radii (up to $\sim$ 0.01 AU).
Tail Dimensions: Modeling indicates the He* tail extends to $\sim$ 40 planetary radii ( $R_p$ ), while the Pa- $\alpha$ tail extends to $\sim$ 19 $R_p$ . The Pa- $\beta$ signal is consistent with the solid body of the planet, likely due to lower optical depth.
Mass-Loss Rate Constraints: The persistence of excited hydrogen (Pa- $\alpha$ ) at large distances implies a dense hydrodynamic outflow. The authors derive a conservative lower limit for the mass-loss rate of $\dot{M} > 10^{13.5}$ g/s.
Composition and Degeneracy Breaking: Standard models with solar He/H ratios failed to simultaneously fit the observed He* and Paschen signals (solar models overpredicted He* absorption at the high mass-loss rates required for Paschen). The data are best explained by a model with a sub-solar He/H ratio ( $\sim$ 3% solar), a mass-loss rate of $10^{13.5}$ g/s, and an initial temperature of 5000 K. The ratio of Pa- $\alpha$ to Pa- $\beta$ absorption further supports a high-density regime where Pa- $\alpha$ is partially saturated.
Stellar Variability: The simultaneous detection of multiple tracers and the long temporal baseline allowed the authors to distinguish planetary absorption from stellar activity, confirming the planetary origin of the extended tail.

Significance and Claims
The paper claims to provide the first simultaneous detection of multiple atmospheric escape tracers (He* and multiple Paschen lines) for a young super-Earth progenitor. The authors assert that these results offer:

Empirical Evidence of Gas-Dwarf Evolution: The detection of vigorous mass loss ( $>10^{13.5}$ g/s) in a 23 Myr-old planet supports the hypothesis that super-Earths and sub-Neptunes lose significant portions of their primordial envelopes during early evolution, sculpting the observed radius valley.
Benchmark for Future Studies: The dataset serves as a legacy benchmark for constraining mass-loss rates in young transiting planets, demonstrating the unique capability of JWST to observe continuously and access multiple tracers simultaneously.
Insight into Escape Mechanisms: While the authors note that distinguishing between photoevaporation and core-powered mass loss requires more data, the high mass-loss rate is consistent with both mechanisms operating in concert. The results suggest that if sustained, V1298 Tau c could lose its entire envelope within the next 10 Myr, transforming into a rocky super-Earth.
Methodological Advancement: The study demonstrates that simultaneous multi-tracer observations are critical for breaking degeneracies in atmospheric models (specifically between mass-loss rate, temperature, and composition) and for characterizing the complex geometry of escaping atmospheres in young systems.

The authors remain modest regarding the specific driving mechanism (photoevaporation vs. core-powered), noting that a single planet is insufficient to empirically distinguish between them, but emphasize that the derived mass-loss rate provides a crucial anchor point for evolutionary models at 23 Myr.

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