Rice team improves models that will detect magnetospheres in distant solar systems
HOUSTON – (June 22, 2021) – We cannot discover them yet, but radio signals from distant solar systems could provide valuable information about the properties of their planets.
An article by Rice University scientists describes a way to better determine which exoplanets are most likely to generate detectable signals based on magnetospheric activity on exoplanets’ previously reduced night sides.
The study by Rice alumnus Anthony Sciola, who received his Ph.D. this spring and was supervised by co-author and space plasma physicist Frank Toffoletto, shows that the radio emissions from the day side of exoplanets seem to be at their maximum when solar activity is high, while those from the night side probably amplify the signal considerably.
This is of interest to the exoplanet community because the strength of a particular planet’s magnetosphere indicates how well it would be protected from the solar wind radiating from its star, just as the Earth’s magnetic field protects us.
Planets orbiting within the Goldilocks zone of a star, where life could otherwise arise, could be considered uninhabitable without evidence of a sufficiently strong magnetosphere. Magnetic field strength data would also help model the interior of planets and understand how planets form, Sciola said.
The study appears in The Astrophysical Journal.
The Earth’s magnetosphere is not exactly a sphere; It is a comet-shaped set of field lines that compress against the day side of the planet and wander into space on the night side, creating eddies, especially during solar events such as coronal mass ejections. The magnetosphere around each planet emits what we interpret as radio waves, and the closer a planet orbits the sun, the stronger the emissions.
Astrophysicists have a pretty good understanding of the planetary magnetospheres of our own system based on Bodes’ law of radiometric, an analytical tool used to establish a linear relationship between the solar wind and the radio emissions of the planets on its way. In recent years, researchers have attempted to apply the law to exoplanetary systems with limited success.
“The community has used these empirical rule-of-thumb models based on what we know about the solar system, but it’s kind of averaged and smoothed,” Toffoletto said. “A dynamic model that has all of this prickly behavior could mean that the signal is actually much larger than these old models suggest. Anthony takes this and pushes it to the limit to understand how signals from exoplanets could be detected. “
Sciola said the current analytical model relies primarily on emissions that are expected to arise from the polar region of an exoplanet, which we see on Earth as the aurora. The new study attaches a numerical model to those estimating polar region emissions to provide a more complete picture of emissions around an entire exoplanet.
“We’re adding features that only appear in lower regions when there is really high solar activity,” he said.
It turned out that the emissions on the night side did not necessarily come from a large point like auroras around the North Pole, but from different parts of the magnetosphere. With strong solar activity, the sum of these night pages could increase the planet’s total emissions by at least an order of magnitude.
“They’re very small and sporadic, but when you put them all together, they can have a big impact,” said Sciola, who continues work at Johns Hopkins University’s Applied Physics Laboratory. “You need a numerical model to resolve these events. For this study, Sciola used the Multiscale Atmosphere Geospace Environment (MAGE), which was developed by the Center for Geospace Storms (CGS) at the Applied Physics Laboratory in collaboration with the Rice Space Plasma Physics Group.
“We are essentially confirming the analytical model for more extreme exoplanet simulations, but adding additional details,” he said. “The insight is that we pay more attention to the limiting factors of the current model, but say that in certain situations you can achieve more emissions than this limiting factor suggests.”
He found that the new model works best on exoplanetary systems. “You really have to be far away to see the effect,” he said. It is difficult to say what is going on on a global scale; It’s like trying to see a movie right off the screen. You only get a small piece of it. “
Also, radio signals from an Earth-like exoplanet may never be detectable from the Earth’s surface, Sciola said. “The earth’s ionosphere is blocking them,” he said. “That means we can’t even see Earth’s own radio emission from the ground, even though it’s so close.”
Detecting signals from exoplanets requires either a satellite complex or an installation on the other side of the moon. “That would be a nice, quiet place to build an array that isn’t constrained by Earth’s ionosphere and atmosphere,” said Sciola.
He said the observer’s position with respect to the exoplanet is also important. “The emission is ‘beamed’,” said Sciola. “It’s like a lighthouse: you see the light when you are in the beam of light, but not when you are directly above the lighthouse. A better understanding of the expected angle of the signal will help observers determine if they are able to observe it for a particular exoplanet. “
Co-authors of the article are Rice PhD student Alison Farrish and David Alexander, professor of physics and astronomy and director of the Rice Space Institute, and computer physicist Kareem Sorathia and physicist Viacheslav Merkin of the Johns Hopkins Applied Physics Laboratory.
The National Science Foundation and NASA supported the research.
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Images to download:
Anthony Sciola, a graduate of Rice University, pictured in Kaldidalur (The Cold Valley) in Iceland, has developed a numerical model to improve the analysis of radio signals from exoplanets. Although the tools to obtain such data are not yet available, they could help determine which planets have protective magnetospheres. (Image: Courtesy Anthony Sciola)
Rice University scientists have improved models that could detect magnetospheric activity on exoplanets. The models add data from nighttime activity that could increase the signals by at least an order of magnitude. In this figure, the planet’s star is in the upper left, and the rainbow spots are the intensities of the radio emission, mostly coming from the night side. The white lines are magnetic field lines. (Image: Anthony Sciola / Rice University)
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