Planetary shields will warp under the stellar winds of their dying stars

Any identified life on planets orbiting white dwarf stars has almost certainly evolved after the star’s death, according to a new study from the University of Warwick that reveals the consequences of the intense and furious stellar winds that will strike a planet as its star dies. The research is published in Monthly notices from the Royal Astronomical Society, and lead author Dr Dimitri Veras will present it today (July 21) at the National Astronomy Meeting Online (NAM 2021).

The research is providing new information to astronomers looking for signs of life around these dead stars by examining the impact their winds will have on orbiting planets during the star’s transition to the white dwarf stage. The study concludes that it’s nearly impossible for life to survive cataclysmic stellar evolution unless the planet has an intensely strong magnetic field – or magnetosphere – that can protect it from the worst effects.

In the case of Earth, solar wind particles can erode the protective layers of the atmosphere that protect humans from harmful ultraviolet rays. The Earth’s magnetosphere acts as a shield to deflect these particles through its magnetic field. Not all planets have a magnetosphere, but Earth’s is generated by its iron core, which spins like a dynamo to create its magnetic field.

“We know that the solar wind in the past has eroded the Martian atmosphere, which, unlike Earth, does not have a large-scale magnetosphere. What we did not expect to find is that the wind solar energy in the future could be as damaging even for those planets which are protected by a magnetic field, ”says Dr Aline Vidotto of Trinity College Dublin, co-author of the study.

All stars eventually run out of the available hydrogen that fuels nuclear fusion in their hearts. In the Sun, the core will contract and heat up, causing the star’s outer atmosphere to expand tremendously into a “red giant”. The Sun will then stretch over a diameter of tens of millions of kilometers, engulfing the inner planets, including perhaps the Earth. At the same time, the star’s loss of mass means that it has a weaker gravitational pull, so that the remaining planets move further apart.

During the red giant phase, the solar wind will be much stronger than today and will fluctuate considerably. Veras and Vidotto modeled the winds of 11 different star types, with masses ranging from one to seven times the mass of our Sun.

Their model demonstrated how the density and speed of the stellar wind, combined with an expanding planetary orbit, conspire to alternately shrink and expand a planet’s magnetosphere over time. For a planet to maintain its magnetosphere at all stages of stellar evolution, its magnetic field must be at least a hundred times stronger than Jupiter’s current magnetic field.

The process of stellar evolution also causes a displacement of a star’s habitable zone, which is the distance that would allow a planet to have the right temperature to support liquid water. In our solar system, the habitable zone would move about 150 million km from the Sun, where the Earth is currently positioned, up to 6 billion km, or beyond Neptune. Although an orbiting planet also changes position during giant branch phases, scientists have found that the habitable zone is moving outward faster than the planet, posing additional challenges to any existing life hoping to survive. to the process.

Eventually, the red giant loses all of its outer atmosphere, leaving behind the remains of warm, dense white dwarf. These do not emit stellar winds, so once the star reaches this stage, the danger to the surviving planets is over.

Dr Veras says that “this study demonstrates the difficulty of a planet in maintaining its protective magnetosphere throughout the phases of the giant branch of stellar evolution.”

“One conclusion is that life on a planet in the habitable zone around a white dwarf would almost certainly develop during the white dwarf phase unless that life was able to withstand multiple extreme and sudden changes in its environment. . “

Future missions like the James Webb Space Telescope, due to launch later this year, should tell more about planets orbiting white dwarf stars, especially if planets in their habitable zones exhibit biomarkers that indicate the presence of life, so the study provides valuable context to any potential discoveries.

So far, no terrestrial planet capable of harboring life around a white dwarf has been found, but two known gas giants are close enough to their star’s habitable zone to suggest that such a planet may exist. These planets likely came closer to the white dwarf as a result of interactions with other more distant planets.

Dr Veras adds that “these examples show that giant planets can come very close to the habitable zone. The habitable zone for a white dwarf is very close to the star because they emit much less light than a star similar to the Sun. Dwarfs are also very stable stars because they have no wind. A planet stationed in the white dwarf habitable zone could remain there for billions of years, allowing time for life to develop provided conditions are favorable.