TRAPPIST-1 c: A Closer Look at Its Atmosphere

Seven rocky planets are orbiting the nearby dwarf star TRAPPIST-1. This situation is exciting because it allows scientists to study the Atmospheres of small planets outside our Solar System. The James Webb Space Telescope (JWST) has recently been launched, making it possible to detect gases like carbon dioxide (CO2) in the atmospheres of these distant worlds.

#Observations of TRAPPIST-1 b and TRAPPIST-1 c

Scientists have focused on TRAPPIST-1 b, the closest planet to the star, and they found it likely to be a bare rock without any CO2. In this study, we report on observations of TRAPPIST-1 c, where we detected Thermal Emissions from its dayside. By analyzing this, we calculated a planet-to-star flux ratio, giving us important information about the temperature of TRAPPIST-1 c's dayside.

#Implications of Findings

The temperature we calculated suggests that TRAPPIST-1 c probably does not have a thick atmosphere rich in CO2. Our data ruled out cloud-free mixtures of oxygen and CO2 with various surface pressures. Even a Venus-like atmosphere with clouds was deemed unlikely. This leads us to think that the planet may have a thin atmosphere or possibly no atmosphere at all. The absence of a thick atmosphere hints at a formation history that lacked volatile compounds, indicating it may have less than the amount of water found in Earth's oceans. If this is true for TRAPPIST-1 c, then it may also be the case for other planets in the same system.

#The Mystery of Planetary Atmospheres

Terrestrial exoplanets and their atmospheric compositions are still largely unknown. Factors that determine an atmosphere's makeup include the initial volatile inventory, volcanic activity, and the potential for atmospheric Escape. Planets around M dwarfs, like TRAPPIST-1, may be especially vulnerable to losing their atmospheres over time. To find out if a planet has an atmosphere, we need to study it directly through methods like thermal emission, reflected light, or transmission spectrums.

Our knowledge of atmospheres has mainly come from studying the thermal emissions of planets such as LHS 3844 b and GJ 1252 b. These observations have shown dayside temperatures that suggest minimal heat distribution and no significant atmospheric absorption from CO2. Because of this, studying cooler planets like TRAPPIST-1 c could yield valuable information about atmosphere retention.

#Methodology of Observations

We conducted observations during four different Eclipses of TRAPPIST-1 c using JWST's Mid-Infrared Instrument (MIRI). The observations spanned several days in October and November 2022. Each visit lasted around 192 minutes and included measures taken during and outside of the eclipse. We utilized a specific filter that allows us to capture emissions from around 15 micrometers, where a significant CO2 absorption feature is located. In total, we gathered over 1,000 integrations during these observations.

#Data Reduction

We analyzed and processed the data in several different ways using custom software and publicly available tools. Each method focused on extracting light curves from the observations. We then compared these light curves against a model of the eclipse while accounting for system noise. Ultimately, the results from different analyses provided consistent estimates for eclipse depths.

#Eclipse Light Curve Analysis

The secondary eclipse light curve measured from the four visits displayed a specific pattern. We binned the data to visualize the eclipse more clearly and fitted a model to estimate the final eclipse depth more accurately. We also considered errors from various sources and included these in our uncertainty calculations.

#Brightness Temperature

From the observed eclipse depth, we calculated the brightness temperature of TRAPPIST-1 c. Our measured temperature was found to be significantly cooler than that of some other rocky planets. This finding positions TRAPPIST-1 c between the inner planets of our Solar System, indicating that it is not as hot as Mercury but also not as cold as Venus.

#Exploring Possible Atmospheric Models

We compared our measured brightness with different atmospheric models to see which scenarios matched our findings. This included cloud-free oxygen-dominated models and pure CO2 atmospheres, considering varying surface pressures and compositions.

For hot rocky planets orbiting M-type stars, a mix of oxygen and CO2 is expected as water breaks down over time. We ruled out thick atmospheres and high pressures based on the observed data.

#Comparison with Venus-like Atmospheres

Given that TRAPPIST-1 c receives slightly more insolation than Venus, we wondered if both planets might share similar atmospheric properties. We modeled different Venus-like atmospheric compositions and analyzed their emissions. Our findings indicated that such atmospheres were not likely on TRAPPIST-1 c, although the cloudy cases provided moderate agreement with our data.

#Bare-Rock Surface Comparisons

After modeling various bare-rock surfaces to see how they fit our data, we found that all surfaces could potentially align with our findings. Weathered surfaces, which could be darker due to processes like space weathering, also appeared consistent with our measurements.

#Results from Emission Models

We simulated different emission spectra for TRAPPIST-1 c to compare against our measured eclipse depth. This included bare-rock models, mixtures of oxygen and CO2, and models based on what we know about Venus. We found that all these models provided insights into possible atmospheric conditions.

#Atmospheric Escape Models

We used models to study how TRAPPIST-1 c's atmosphere might have changed over time. By understanding the planet's initial water abundance and how much has escaped, we can better gauge its current state. Our findings suggest that the planet likely began with less water.

#Interior Structure Models

We also looked at the planet's internal structure to understand better its mass, radius, and possible composition. The interior model indicates that TRAPPIST-1 c has a silicate-rich mantle and a core, providing further details about its makeup.

#Stellar Properties and Measurements

Our work also involved analyzing the star TRAPPIST-1 itself. By measuring various properties, we could accurately relate the brightness of the star to that of the planet. This information is crucial for calculating temperature and understanding how the star influences its orbiting planets.

#Eclipse Timing Variations

We monitored how the timing of eclipses could shift compared to predictions. Various factors, such as the light-travel time across the system and the planet's eccentricity, can affect eclipse timings. Our comparisons show that the observed and predicted times align closely, allowing us to refine future studies on the planet's orbit.

#Conclusion

The research around TRAPPIST-1 c offers insights into the possible atmospheres and conditions of rocky exoplanets. The findings indicate a volatile-poor planet that likely formed with limited water and challenges the notion of whether smaller planets can hold on to significant atmospheric gases, especially around M dwarf stars. Further observations of the other planets in the TRAPPIST-1 system will be vital in understanding the habitable zone and the potential for life outside our Solar System.