According to a new study done by the University of Warwick, any life discovered on planets orbiting white dwarf stars almost probably evolved after the star’s demise. The study explores the effects of the powerful stellar winds that will pummel a planet when its star is dying. Lead researcher Dr. Dimitri Veras will present the study today (July 21, 2021) at the online National Astronomy Meeting (NAM 2021). It has been published in Monthly Notices of the Royal Astronomical Society.
By analyzing the effect that these dead stars’ winds will have on orbiting planets throughout their transition to the white dwarf stage, the research offers new information for astronomers looking for indications of life around these dead stars.
The study finds that unless a planet has a magnetosphere that can protect it from the worst effects, it is almost difficult for life to survive cataclysmic stellar evolution.
Solar wind particles on Earth have the potential to destroy the layers of the atmosphere that screen people from dangerous UV radiation. Through its magnetic field, the terrestrial magnetosphere acts as a shield to deflect such particles.
The magnetosphere of Earth, which is not present on other planets, is produced by the iron core of the planet, which rotates like a dynamo to produce the magnetic field.
“We know that the solar wind in the past eroded the Martian atmosphere, which, unlike Earth, does not have a large-scale magnetosphere. What we were not expecting to find is that the solar wind in the future could be as damaging even to those planets that are protected by a magnetic field,” says Dr. Aline Vidotto of Trinity College Dublin, the co-author of the study.
Every star eventually runs out of hydrogen, which is used to power nuclear fusion in their centres. The Sun’s core will soon begin to shrink and heat up, which will cause the star’s outer atmosphere to rapidly expand and turn into a “red giant.”
The Sun will eventually enlarge to tens of millions of kilometers in diameter, devouring the inner planets, perhaps even the Earth. The remaining planets migrate further away from the star as a result of its decreased gravitational pull due to the loss of mass in the star.
This study demonstrates the difficulty of a planet maintaining its protective magnetosphere throughout the entirety of the giant branch phases 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.
Dr. Dimitri Veras
The solar wind will be much stronger and more erratic during the red giant phase than it is now. With masses ranging from one to seven times that of the Sun, Veras and Vidotto modelled the winds from 11 different types of stars.
Their simulation showed how a planet’s magnetosphere might alternately grow and shrink over time depending on the density and speed of the stellar wind and an expanding planetary orbit.
Any planet’s magnetic field must be at least 100 times stronger than Jupiter’s present magnetic field in order to sustain its magnetosphere during all phases of star evolution.
The habitable zone of a star, which is the distance at which a planet may be at the proper temperature to support liquid water, shifts as a result of stellar evolution. The habitable zone in our solar system would expand from the location of Earth, which is currently 150 million kilometers from the Sun, up to 6 billion kilometers, or beyond Neptune.
The scientists discovered that the habitable zone spreads outward more fast than the planet, offering extra obstacles to any existing life attempting to survive the process. This is true even though an orbiting planet would likewise change position during the big branch phases.
The dense hot white dwarf remnant is left behind when the red giant eventually sheds all of its outer atmosphere. Once the star reaches this stage, the threat to surviving planets has vanished because these stars do not emit stellar winds.
Dr. Veras said: “This study demonstrates the difficulty of a planet maintaining its protective magnetosphere throughout the entirety of the giant branch phases 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.”
The study gives important context to any potential discoveries and should help future missions, such as the James Webb Space Telescope, which is scheduled to launch later this year, learn more about planets that orbit white dwarf stars, including whether planets within their habitable zones show biomarkers that indicate the presence of life.
Although no terrestrial planet that potentially harbor life has been discovered yet in the region of a white dwarf, two known gas giants are sufficiently close to their star to raise the possibility that such a planet might exist. These planets probably migrated toward the white dwarf because to interactions with planets farther away.
Dr. Veras adds: “These examples show that giant planets can approach 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 Sun-like star.”
“However, white dwarfs are also very steady stars as they have no winds. A planet that’s parked in the white dwarf habitable zone could remain there for billions of years, allowing time for life to develop provided that the conditions are suitable.”
