The First Dedicated Survey of Atmospheric Escape from Planets Orbiting F Stars

This paper presents the first dedicated survey of atmospheric escape from gas giants orbiting F stars using Palomar/WIRC observations, revealing strong and tentative helium detections in several systems and demonstrating that their mass-loss rates are best explained by a combination of Roche filling factor and XUV luminosity rather than NUV-driven models.

The Gist

Imagine the universe as a vast, cosmic ocean. In this ocean, stars are like lighthouses, and planets are like boats sailing nearby. Usually, we think of these boats as sturdy vessels, but when a planet gets too close to its star, the star's intense radiation acts like a giant, invisible hair dryer. This "hair dryer" blasts the planet's atmosphere, stripping away gas and sending it streaming out into space like steam from a boiling kettle. This process is called atmospheric escape.

For a long time, astronomers have been studying this "steam" coming off planets orbiting cool, red stars (like M-dwarfs). But what about planets orbiting hotter, brighter, blue-white stars (called F-stars)? Do they lose their atmospheres faster? Do they have bigger, more dramatic "steam clouds"?

This paper is the first dedicated survey to answer that question. The authors, led by Morgan Saidel, decided to take a closer look at six specific planets orbiting F-stars to see how their atmospheres are behaving.

The Mission: A Cosmic "Heads-Up" Detector

To see this escaping gas, the team used a special tool: the Palomar/WIRC telescope equipped with a very specific filter. Think of this filter like a pair of sunglasses that only lets through a very specific color of light emitted by helium (a gas that makes up a big part of these planets' atmospheres).

When a planet passes in front of its star (a transit), the starlight usually dips slightly because the planet blocks it. But if the planet has a giant cloud of escaping helium, the light dips even more because the gas absorbs some of the starlight too. By measuring this extra dip, the team can tell how much gas is escaping.

The Results: A Mixed Bag of Steam Clouds

The team observed six planets. Here is what they found, using some simple analogies:

1. The "Geyser" Planets (WASP-12 b & WASP-180 A b):
   Two of the planets showed strong, clear signs of massive atmospheric escape.

   • WASP-12 b is like a planet that is literally being torn apart. It orbits so close to its star that it's almost touching the edge of its own gravitational "bubble" (called the Roche lobe). The authors found it has a huge, powerful outflow, confirming it's losing mass at a rate of about 10 trillion grams per second.
   • WASP-180 A b was a surprise. It doesn't orbit as dangerously close as WASP-12, but its star happens to be very active and "sunny" in high-energy radiation. This extra radiation acted like a turbocharger, driving a strong escape of gas.

2. The "Faint Whispers" (WASP-93 b & HAT-P-8 b):
   Two other planets showed weak hints of escaping gas. It's like hearing a faint whisper in a noisy room. The signal was there, but not strong enough to be 100% sure without more observations.

3. The "Dry Rocks" (WASP-103 b & KELT-7 b):
   The other two planets showed no signs of escaping gas at all. Despite being hot Jupiters (giant gas planets), their atmospheres seemed stable. This was a surprise, especially for WASP-103 b, which looked like it should be losing gas based on its size and distance.

The Big Discovery: It's Not Just About the Star's Heat

Before this study, scientists noticed that some planets around hot stars had massive, dramatic outflows. They wondered: "Is it because hot stars just naturally blow harder?"

The answer from this survey is a resounding no.

The team found that not all planets around hot stars are losing their atmospheres at the same crazy rate. In fact, most of the planets they studied were losing gas at a "normal" pace, similar to planets around cooler stars.

So, what makes the difference? The authors identified two main "ingredients" that create a massive atmospheric storm:

• The "Roche Filling" Factor: Imagine a planet is a balloon. If the balloon is so big it's almost touching the walls of the room it's in, it's easy for air to leak out. Planets that are physically large and orbit very close (filling up their "gravitational room") are much more likely to have massive outflows.
• The "Star's Mood" (XUV Radiation): Even if a planet is in a dangerous spot, it needs a strong push to escape. This push comes from the star's high-energy radiation (X-rays and extreme UV). The study found that the stars with the most "moody" and energetic radiation produced the strongest planetary outflows.

Why This Matters

This study is like realizing that not every house near a volcano is going to be destroyed. Some are safe, some are damaged, and some are obliterated, depending on exactly where they are and how the volcano is behaving that day.

The team also solved a mystery regarding WASP-12 b. Other telescopes had looked at this planet before and didn't see the escaping gas. The authors suggest that the gas escaping WASP-12 might be forming a giant, rotating ring (a torus) around the star. Sometimes this ring blocks the view, and sometimes it doesn't. It's like trying to see a lighthouse through a thick fog that comes and goes; one night you see the light, the next night you don't.

The Bottom Line

This survey tells us that hot stars don't automatically strip their planets' atmospheres. Instead, a "perfect storm" is needed: the planet must be huge and close to the star, and the star must be blasting it with extra high-energy radiation. Without both, the planet's atmosphere can survive, even in the harsh environment of a hot star system.

This helps us understand how planets evolve over billions of years and why some become rocky worlds while others disappear entirely.

Technical Summary

Here is a detailed technical summary of the paper "The First Dedicated Survey of Atmospheric Escape from Planets Orbiting F Stars" by Saidel et al. (2026).

1. Problem Statement and Motivation

While hydrodynamic escape is known to strip the envelopes of close-in exoplanets, the majority of observational evidence for atmospheric mass loss has been confined to planets orbiting cooler K and M dwarfs. A limited sample of planets orbiting early-type (F and A) stars has shown unusually strong and extended outflows, with mass-loss rates exceeding $10^{12}$g s$^{-1}$.

However, it remains unclear whether these strong outflows are representative of the general population of gas giants orbiting F stars or if they are unique to specific systems. Three competing hypotheses exist to explain the high mass-loss rates in early-type systems:

1. Roche Lobe Overflow (RLO): Planets filling a large fraction of their Roche lobes experience enhanced mass loss.
2. Enhanced XUV Flux: Early-type stars may possess higher X-ray and Extreme Ultraviolet (XUV) luminosities that drive stronger outflows.
3. NUV-Driven Escape: High Near-Ultraviolet (NUV) fluxes from F stars could drive Balmer-line escape, potentially increasing mass loss rates by orders of magnitude compared to EUV-driven models.

To resolve this, the authors initiated the first dedicated survey of atmospheric escape from gas giants orbiting F stars to characterize their outflow properties and determine the dominant drivers of mass loss.

2. Methodology

Target Selection

The team selected six transiting gas giant planets orbiting F stars ($T_{eff} = 6000–7935$ K) observable from Palomar Observatory. Targets were chosen to span a range of:

• Roche filling factors ($R_p/R_{Roche}$): 0.20 – 0.55.
• Projected stellar rotational velocities ($v \sin i_*$): 2 – 70 km s$^{-1}$ (used as a proxy for XUV activity).
• NUV fluxes.
  The survey targets were: HAT-P-8 b, WASP-93 b, WASP-180 A b, WASP-103 b, KELT-7 b, and WASP-12 b.

Observations

• Instrument: Palomar/WIRC (Wide-field Infrared Camera).
• Filter: Ultra-narrowband filter centered on the metastable helium (He*) line at 1083.3 nm (FWHM: 0.635 nm).
• Data: Ten transit observations were obtained between 2023 and 2025.
• Reduction: Standard photometric reduction including dark subtraction, flat-fielding, and sky background correction. Telluric water absorption and OH emission were corrected using dithered background frames and scaling factors.
• Light Curve Modeling: Transits were modeled using exoplanet and starry. Systematics were detrended using a combination of airmass, PSF width, and a time-varying telluric water proxy, selected via Bayesian Information Criterion (BIC). Limb darkening coefficients were treated as either free parameters or fixed based on consistency with ldtk model predictions.

Mass Loss Modeling

Measured He* excess absorption depths were converted into mass-loss rates ($\dot{M}$) using the sunbather package.

• Model: 1D isothermal Parker wind models.
• Stellar Spectra: Proxy stellar spectra from the MUSCLES survey were used. For targets with published XUV constraints (WASP-180 A, KELT-7, WASP-12), the proxy fluxes were scaled to match observed XUV luminosities. For others, a range of XUV fluxes (from proxy values up to HAT-P-32 levels) was tested to quantify uncertainty.
• Grid Search: A grid of mass-loss rates ($10^9$–$10^{13}$g s$^{-1}$) and thermosphere temperatures (5000–11,000 K) was computed to find self-consistent solutions.

3. Key Results

Detection Statistics

The survey yielded the following detections of atmospheric escape (He* excess absorption):

• Strong Detections (>3$\sigma$):
  • WASP-12 b: $0.73 \pm 0.13%$excess depth ($5.6\sigma$).
  • WASP-180 A b: $0.56 \pm 0.15%$excess depth ($3.7\sigma$).
• Tentative Detections (2–3$\sigma$):
  • HAT-P-8 b: $0.82 \pm 0.30%$excess depth ($2.7\sigma$).
  • WASP-93 b: $0.22 \pm 0.11%$excess depth ($2.0\sigma$).
• Non-Detections:
  • WASP-103 b: No significant excess absorption (Upper limit: $<0.018\%$).
  • KELT-7 b: No significant excess absorption (Upper limit: $<0.018\%$).

Derived Mass-Loss Rates

• WASP-12 b: $\log \dot{M} \approx 12.4$ g s$^{-1}$ ($\sim 2.5 \times 10^{12}$ g s$^{-1}$).
• WASP-180 A b: $\log \dot{M} \approx 11.85$ g s$^{-1}$ ($\sim 7 \times 10^{11}$ g s$^{-1}$).
• Other Targets: Mass-loss rates for HAT-P-8 b, WASP-93 b, WASP-103 b, and KELT-7 b are significantly lower (mostly $< 10^{11}$ g s$^{-1}$), consistent with rates observed for planets orbiting cooler stars.

Analysis of Drivers

• Roche Filling Factor vs. XUV: The planets with the highest mass-loss rates (WASP-12 b, WASP-180 A b) possess either high Roche filling factors (WASP-12 b: 0.55) or exceptionally high XUV luminosities (WASP-180 A b).
• The WASP-103 Anomaly: WASP-103 b has a high Roche filling factor (0.51) comparable to WASP-12 b but showed no detectable outflow. This suggests its host star has a significantly lower XUV flux than expected, or the He* fraction in the outflow is lower than predicted.
• NUV-Driven Escape: No correlation was found between NUV flux and mass-loss rates. This disfavors the hypothesis that Balmer-driven escape (powered by NUV) is the dominant mechanism for these F-star systems.
• WASP-12 b Discrepancy: The strong detection in this work contrasts with previous non-detections by CARMENES (S. Czesla et al. 2024a). The authors propose that a time-varying circumstellar torus of gas (fed by the planet's outflow) may have obscured the signal in previous observations, or that baseline subtraction in the CARMENES data (which had a shorter baseline) masked the extended signal.

4. Significance and Conclusions

1. Population Heterogeneity: The survey demonstrates that strong atmospheric outflows ($\dot{M} > 10^{12}$ g s$^{-1}$) are not a universal characteristic of all gas giants orbiting F stars. Most surveyed planets exhibit mass-loss rates comparable to those around cooler stars.
2. Dominant Drivers: The variation in mass-loss rates is best explained by a combination of Roche filling factor and stellar XUV luminosity. High XUV flux is critical; effective temperature and stellar rotation ($v \sin i_*$) are poor proxies for XUV activity in these systems.
3. Refutation of NUV-Driven Dominance: The lack of correlation with NUV flux suggests that Balmer-driven escape is not the primary driver for the majority of these systems, challenging previous theoretical predictions.
4. Observational Implications:
   • 3D Geometry: Theoretical modeling suggests all surveyed planets likely have "stream-like" or tail-like outflow morphologies (low Rossby numbers) rather than spherical bubbles. This implies that significant absorption may occur outside the nominal transit window, potentially biasing photometric measurements if baselines are not extended.
   • Time Variability: The discrepancy in WASP-12 b highlights the potential for temporal variability in circumstellar environments, necessitating long-term monitoring to fully characterize mass loss in systems with high outflow rates.

This work establishes that while F-star planets can host extreme outflows, these are specific to systems with favorable conditions (high Roche filling and/or high XUV), rather than a general property of the spectral type. Future spectroscopic observations with extended baselines are required to fully constrain the 3D geometry and kinematics of these outflows.