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The team models the star activity of the Milky Way's neighbors to assess planetary impact – ScienceDaily



The exoplanet may seem like the perfect place to take care of your home, but before you go there, take a closer look at its star.

Rice University astrophysicists are doing just that by building a computer model to help assess how the star's own atmosphere affects her planets, for better or worse.

By narrowing living conditions, they hope to refine their search for potentially habitable planets. Astronomers now suspect that most of the billions of stars in the sky have at least one planet. So far, observers from Earth have noticed almost 4,000 of them.

Rice's main author and graduate, Alison Farrish and her research advisor, solar physicist David Alexander, conducted the first group study to characterize the "cosmic weather" environment of stars other than ours to see how this would affect magnetic activity around the exoplanet. This is the first step in a National Science Foundation-funded study of magnetic fields around the planets themselves.

"With current technology, it is not possible to determine whether an exoplanet has a protective magnetic field or not, so this article focuses on what is known as an asterospheric magnetic field," Farrish said. "This is an interplanetary extension of the stellar magnetic field that the exoplanet would interact with."

In a study published in The Astrophysical Journal, researchers are extending the magnetic field model that combines what is known about the transport of a magnetic solar stream – the movement of magnetic fields around, through and emanating from the surface of the sun – to a wide range of stars with different levels of magnetic activity. The model is then used to create a simulation of the interplanetary magnetic field surrounding these simulated stars.

In this way, they were able to hypothesize the potential environment experienced by such "popular" exoplanet systems, such as Ross 128, Proxima Centauri and TRAPPIST 1, all stunted stars with known exoplanets.

No star is ever calm. The plasma on its surface is constantly storming, causing disturbances that emit strong magnetic fields (such as those embedded in the solar wind) far into space. Earth's own magnetosphere helps to make it a safe haven for life, but whether this is the case for every exoplanet, it remains to be determined.

"For most people, the planet 'habitable zone' traditionally means it has the right temperature for liquid water," Farrish said. "But in these particular systems, the planets are so close to their stars that there are other considerations. In particular, magnetic interaction is becoming very important. "

These "Goldilocks" planets may enjoy atmospheric temperatures and pressures that allow life-giving water to exist, but they are probably circling too close to their stars to escape the star's strong magnetic fields and associated radiation.

"It is estimated that depending on where the star's expanded magnetic field is located, some of these residential zone exoplanets may lose their atmosphere in just 100 million years," said Alexander. "It's a very short time in astronomical terms. The planet may have adequate temperature and pressure conditions for housing, and simple life forms may be formed, but this is as far as possible. The atmosphere would be stripped and the surface radiation would be quite intense.

"When you don't have the atmosphere, you now have all the ultraviolet and X-ray emission from the star over the particle emission," he said. "We want to better understand this interaction and be able to compare it with future observations. And the ability to direct and define the nature of these future observations will be really helpful. "

The key parameters in the model are the Rossby star number, which determines how active the star is, and the Alfvén surface, which determines where the asterospheric magnetic field effectively separates the star.

"Our model allows us to identify some of the key features of star activity in relation to the appearance of flux and transport during the star cycle," said Alexander. "This allows direct comparison with observations that are now very rare for stars other than the Sun, and a means by which some key physical attributes of exoplanets can potentially be characterized by their interaction with the star field."

"All planetary systems that people are currently paying close attention to – Ross, Proxima and TRAPPIST – are popular because they have planets in their habitable zones, but, according to our calculations, most of them fit into Alfven's surface area," Farrish said . "This creates the potential for a direct magnetic connection between the star and the planet, which would more strongly cause a loss of the planet's atmosphere."

One such planet revolves around Proxima Centauri. "The star is the size of the seventh-seventh, and the planet is 20 times closer," said Alexander. "It's good for temperature, but bad for magnetic conditions." Farrish and Alexander note that the team found one unique arrangement in GJ 3323, the M dwarf, which contains two "super Earths" discovered in 2017. One, GJ 3323 b, lies in the star's living zone, but also in its surface, Alfvéna. The second, GJ 3323 c, circulates beyond the Alfvén surface, but unfortunately far beyond the residential zone.

"I think not to say that there is one system of which we are all excited, but having two planets of the same age on both sides of Alfvén's surface may prove useful when observations improve, in studying how fields are formed magnetic in exoplanets – said Alexander.


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