Mysteries of the gas giants known as ‘hot Jupiters’ solved

A major new study led by UCL researchers answers five longstanding questions about planets outside our solar system, known as “hot Jupiters.”

Artist's impression of 25 hot Jupiters

The study, published in The Astrophysical Journal Supplement Series, is one of the largest surveys of exoplanet atmospheres ever conducted. The researchers employed high-performance computers at the DIRAC HPC facility to analyze the atmospheres of 25 hot Jupiters using data from nearly 1,000 hours of telescope observations.

Hot Jupiters are gas giants that orbit close to their star, typically in less than 10 days. While there are none in our own solar system, they are the most commonly seen types of planets outside it.

By using a large sample of exoplanets and analyzing a very large amount of data, the researchers were able to determine trends and address questions that smaller studies have been unable to answer conclusively for many years.

Among the findings, the researchers found that the night and day sides of hot Jupiters varied greatly, with temperatures dropping by hundreds of degrees centigrade from day to night (the researchers found a 1000K difference on average).

They also found that many hot Jupiters had thermally inverted atmospheres, also known as stratospheres—that is, their upper atmospheres with temperatures that increased with altitude. This appears to be due to the presence of metallic elements (titanium oxide, vanadium oxide and iron hydride), which absorb the star’s light and thus heat the atmosphere. This is a similar phenomenon that occurs through the ozone layer on Earth.

The researchers also confirmed that the molecules were breaking up in the hottest planets; They found that some of the planets contained less water than expected, suggesting that they formed in a different way to the more water-abundant planets; And they detected more metals than predicted by the models, meaning those planets were likely different than previously thought.

A better understanding of exoplanets will help solve questions about the evolution of our own solar system, the researchers said.

Lead author Dr Quentin Changet (UCL Physics and Astronomy) said: “Many issues such as the origin of water on Earth, the formation of the Moon, and the different evolutionary histories of Earth and Mars are still unresolved, despite our ability to obtain In-situ measurements. Larger exoplanet population studies, such as we present here, aim to understand those common processes.”

Co-lead author Dr. Billy Edwards (UCL Physics and Astronomy and Commissariat a l’Energie Atomique et aux Energies Choices, or CEA) said: “Our paper marks a turning point for the field. We are now from the characterization of individual exoplanet atmospheres. Moving towards characterization of atmospheric populations.”

The research team used 600 hours of observations from the NASA/ESA Hubble Space Telescope and 400 hours of data from the Spitzer Space Telescope. They combined two techniques – the study of information from transits (where the planet passes in front of its star) and eclipses (when the planet passes behind its star). The tools he created were open source and are now available to researchers all over the world.

This work was made possible by the collaborative use of large supercomputing facilities: the UK’s DIRAC High Performance Computing (HPC) facility, funded by the Science and Technology Facilities Council (STFC), and the OZSTAR facility in Swinburne, Australia. Significant computing resources were required because each planet observation had to be tested against millions of atmospheric models to extract the best possible scenarios.

Dr Ahmed Al-Refai, Head of Numerical Methods at the UCL Center for Space Exochemistry Data and co-author of the study, said: “The need for state-of-the-art equipment and supercomputing resources is paramount in making this type of large-scale analysis possible. The exo-atmosphere, in particular, is heading towards the era of the James Webb Space Telescope and ESA’s Aerial Space Mission.

The study involved an international team of researchers from UCL, Queen Mary University of London, University of Exeter, CEA, LISA, University of Zurich, University of Florence, the Flatiron Institute in the US and the National Astronomical Observatory in Japan. NAOJ) and The Graduate University for Advanced Studies, Sokendai, Japan. The research was supported by UKRI, STFC, the UK Space Agency, the European Research Council, CNES (France), CNRS (France), ANR (France) and the Japan Society for the Promotion of Science.

  • Artist’s impression of 25 Hot Jupiters. credit: ESA/Hubble, n. bartman
  • /public release. This material from the parent organization/author(s) as may be of a periodic nature, has been edited for clarity, style and length. The views and opinions expressed are those of the author(s). View full here.


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