How a Cosmic Thermometer Could Reveal the Secrets of Exoplanets

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31 Aug 2023
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Have you ever wondered how hot or cold an exoplanet is? How about whether it has the right conditions for life? If so, you might be interested in a new discovery that could help answer these questions.

Image Source: TheInsightPost


A team of astronomers, led by Cornell University, has detected a molecule called chromium hydride (CrH) in the atmosphere of a hot Jupiter exoplanet called WASP-31b. This molecule is very sensitive to temperature and is only abundant in a narrow range between 1,200 and 2,000 degrees Kelvin. This means that it could act as a “cosmic thermometer” for exoplanets, allowing scientists to measure their temperature and other characteristics.

What is WASP-31b and why is it special?

WASP-31b is a gas giant planet that orbits very close to its host star, an F5-type star that is slightly hotter and larger than our Sun. It was discovered in 2011 by the Wide Angle Search for Planets (WASP) project, which uses ground-based telescopes to look for transits of exoplanets. A transit is when a planet passes in front of its star, blocking some of its light and causing a small dip in its brightness.

WASP-31b has some unusual features that make it an interesting target for exoplanet research. For one thing, it has an extremely low density, even for a giant planet. It is about 1.5 times the size of Jupiter, but only 0.48 times its mass. This means that it has a very puffy atmosphere that extends far from its surface.

Another feature of WASP-31b is that it has a highly eccentric orbit, meaning that its distance from its star varies significantly during each revolution. This causes its temperature to change dramatically as well, from about 1,100 K at its farthest point to about 1,700 K at its closest point. This variation in temperature could affect the chemistry and dynamics of its atmosphere, creating different patterns of clouds and winds.

How did the astronomers detect chromium hydride on WASP-31b?

The astronomers used a technique called high-resolution spectroscopy to detect and analyze the atmosphere of WASP-31b. This technique involves splitting the light from the star and the planet into its component wavelengths, like a prism, and looking for specific signatures of molecules that absorb or emit light at certain frequencies.

The astronomers used two instruments to collect high-resolution spectra of WASP-31b: the Gemini Remote Access to CFHT ESPaDOnS Spectrograph (GRACES) on the Gemini North telescope in Hawaii, and the High Accuracy Radial velocity Planet Searcher (HARPS) on the ESO 3.6-meter telescope in Chile. They observed WASP-31b during two transits and two secondary eclipses, when the planet passes behind its star.

By comparing the spectra from different phases of the orbit, the astronomers were able to isolate the contribution of the planet’s atmosphere from that of the star. They then used a method called singular value decomposition (SVD) to enhance the signal-to-noise ratio and extract the molecular features of interest.

Among the molecules they detected on WASP-31b, chromium hydride stood out as the most surprising and significant one. Chromium hydride has never been confirmed on any exoplanet before, and it is also very rare in our own solar system. The only place where it has been detected is in sunspots, where the temperature is lower than the rest of the Sun’s surface.

Chromium hydride is also very sensitive to temperature, as it forms or breaks apart depending on how hot or cold the environment is. At hotter temperatures, chromium atoms are ionized or exist as free atoms, while at lower temperatures they form other compounds with different elements. Only in a specific temperature range, between 1,200 and 2,000 K, does chromium hydride become abundant enough to be detectable.

This means that chromium hydride could act as a “cosmic thermometer” for exoplanets, allowing scientists to measure their temperature and infer other properties such as their pressure, composition, and cloud cover. For example, by comparing the amount of chromium hydride at different phases of WASP-31b’s orbit, the astronomers were able to confirm its equilibrium temperature at 1,400 K, which agrees with previous estimates based on other methods.

What are the implications and challenges of this discovery?

The discovery of chromium hydride on WASP-31b is an important milestone in exoplanet research, as it demonstrates the power and potential of high-resolution spectroscopy to reveal new information about distant worlds. It also opens up new possibilities for using metal hydrides as probes of exoplanet atmospheres, as they are expected to be more common and diverse than previously thought.

Metal hydrides are molecules that consist of a metal atom bonded to a hydrogen atom, such as iron hydride (FeH), titanium hydride (TiH), or magnesium hydride (MgH). They are usually found in cool stars and brown dwarfs, but they could also exist in hot Jupiter exoplanets, especially those that have migrated close to their stars and lost some of their original hydrogen and helium.

Metal hydrides have different temperature sensitivities and spectral features, which could help scientists to constrain the temperature, pressure, and composition of exoplanet atmospheres. They could also provide clues about the formation and evolution of exoplanets, as they reflect the metallicity and history of their host stars.

However, there are also some challenges and limitations to using metal hydrides as exoplanet thermometers. One challenge is that they are often blended with other molecular lines or obscured by clouds, making them difficult to detect and identify. Another challenge is that they depend not only on temperature, but also on other factors such as the amount of hydrogen available, the mixing and transport of gases, and the presence of other elements or molecules that could react with them.

Therefore, to fully exploit the potential of metal hydrides as exoplanet thermometers, scientists need to develop more sophisticated models and methods that can account for these complexities and uncertainties. They also need to collect more high-resolution spectra of exoplanets with different types of stars, orbits, and atmospheres, to test and refine their theories and predictions.

What does this discovery mean for the search for life?

The discovery of chromium hydride on WASP-31b does not imply that this planet is habitable or has life. In fact, WASP-31b is very unlikely to support life as we know it, as it is too hot, too large, and too exposed to radiation from its star. However, this discovery does have some implications for the search for life on other exoplanets.

One implication is that metal hydrides could help us to identify potentially habitable exoplanets, by narrowing down the range of temperatures where liquid water could exist on their surfaces. Liquid water is considered to be a key ingredient for life as we know it, so finding exoplanets in the habitable zone of their stars is one of the main goals of astrobiology.

Another implication is that metal hydrides could help us to characterize the atmospheres of potentially habitable exoplanets, by revealing their pressure, composition, and cloud cover. These factors could affect the climate, stability, and biosignatures of these planets, which are indicators of possible life.

For example, metal hydrides could tell us if an exoplanet has a thick or thin atmosphere, which could influence its greenhouse effect and surface temperature. They could also tell us if an exoplanet has a high or low metallicity, which could reflect its origin and evolution. They could also tell us if an exoplanet has clouds or hazes, which could affect its albedo and radiation balance.

However, metal hydrides are not the only or the best way to study the habitability and biosignatures of exoplanets. There are many other molecules and methods that could provide complementary or more conclusive information about these aspects. For example, water vapor (H2O), carbon dioxide (CO2), methane (CH4), oxygen (O2), ozone (O3), and nitrous oxide (N2O) are some of the most important molecules for habitability and biosignatures, as they are related to the presence and activity of life on Earth.

Therefore, to fully assess the habitability and biosignatures of exoplanets, scientists need to combine multiple techniques and observations from different instruments and missions. Some of these include the Transiting Exoplanet Survey Satellite (TESS), which is currently searching for transiting exoplanets around nearby stars; the James Webb Space Telescope (JWST), which will launch in 2023 and will be able to observe the spectra of exoplanet atmospheres in infrared; and the Extremely Large Telescope (ELT), which will be completed in 2025 and will be able to observe the spectra of exoplanet atmospheres in visible and near-infrared.

Conclusion

WASP-31b is a fascinating exoplanet that has revealed a new molecule in its atmosphere: chromium hydride. This molecule is very rare and temperature-sensitive, making it a useful tool for measuring the temperature and other properties of exoplanets. By using high-resolution spectroscopy, astronomers were able to detect and analyze chromium hydride on WASP-31b, confirming its equilibrium temperature and opening up new avenues for exoplanet research.

However, detecting chromium hydride on exoplanets is not easy or straightforward, as it depends on many factors and uncertainties. To fully exploit its potential as a cosmic thermometer, astronomers need more data and models to account for these complexities. They also need to combine it with other molecules and methods that can provide more information about the habitability and biosignatures of exoplanets.

The discovery of chromium hydride on WASP-31b is an exciting achievement that demonstrates the power and potential of high-resolution spectroscopy for exoplanet research. It also shows that metal hydrides are more common and diverse than previously thought, and that they could help us to explore and understand the diversity and complexity of exoplanet atmospheres.

What are your thoughts on this discovery? Do you think metal hydrides could help us find more habitable exoplanets in the future? 🌎🚀


I hope you like this article, and learn something new. If you have any question or comments, please feel free to share them with me. I would love to hear from you. And if you want to learn more exoplanets, astronomy, A.I and other science topics, please check out our other articles and resources.



📚 Sources:


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(2) “Cosmic Thermometer” Detected on WASP-31b - SciTechDaily | SciTechDaily.
(3) 'Thermometer' molecule confirmed on exoplanet WASP-31b - Phys.org | Phys.org.
(4) ‘Thermometer’ Molecule Confirmed on Exoplanet WASP-31b | Technology.org.
(5) Evidence for substance at liquid-gas boundary on exoplanet WASP-31b | Phys.org.
(6) ‘Thermometer’ molecule confirmed on exoplanet WASP-31b | Cornell University.
(7) Temperature of a rocky exoplanet measured -- ScienceDaily | ScienceDaily.
(8) Overview | What is an Exoplanet? – Exoplanet ... - Exoplanet Exploration | Exoplanet Exploration.
(9) rocky exoplanet classification method and its application to .... | Oxford Academic.
(10) NASA SVS | What Makes an Exoplanet Habitable? | NASA SVS.
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(12) Planetary habitability - Wikipedia | Wikipedia.
(13) Exoplanets: Criteria for their Habitability and Possible Biospheres .... | Springer.
(14) What exactly makes an exoplanet 'habitable? | Digital Trends | Digital Trends.
(15) The Habitability of Exoplanets - University of Groningen | University of Groningen.

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