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Thank God for the Sun: Massive Stars are Less Habitable

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The Last Universal Common Ancestor for life-as-we-know-it was a complex microbe that may have thrived in the shallow waters of early Earth, when the Moon loomed bigger in the sky as it was closer to Earth. (Image credit: Mark Garlic/Science Source)

A new paper that I co-authored with my brilliant postdoc, Devesh Nandal (available here), suggests gratitude to our cosmic fortunes.

Put aside your frustrations with terrestrial geopolitics or complaints about your fellow earthlings. Look up at the brightest stars in the night sky. According to the new paper, the real estate near these massive stars has a low value for life in our cosmic neighborhood of the Milky-Way galaxy.

As residents of Earth, we are very fortunate to live next to the Sun. This star provided the appropriate amount of heat to maintain life-as-we-know-it for 4.2 billion years. This lifespan was inferred from our Last Universal Common Ancestor, a microbe which enjoyed a diet of hydrogen and carbon dioxide on early Earth, as reported here.

The flux of light suitable to support liquid water on the surface of a rocky planet with an atmosphere, defines the habitable zone around any star. In the vicinity of faint dwarf-stars, the habitable zone is closer in. For our nearest red-dwarf neighbor, Proxima Centauri, the habitable zone is about 20 times closer than it is for the Sun. In 2016, a rocky planet was discovered at that separation and named Proxima b. Habitable-zone planets are common around dwarf stars, which are the most abundant and longest-lived population of stars.

This raises a major paradox that I discussed in a 2014 paper, published here. Dwarf stars account for about 90% of the stars in the Milky-Way galaxy and the least massive stars among them — down to 0.07 solar masses, outlive the Sun by up to a factor of a thousand. Given these facts: why do we live next to the Sun today rather than next to a common dwarf star in the future?

The likely explanation is that dwarf stars, like Proxima Centauri, have winds and flare frequently. Since the habitable zone is closer in, their winds can strip the atmospheres of habitable planets and their intense UV and X-rays flares can sterilize any surface life (as discussed in a 2017 paper I co-authored with my postdoc then, Manasvi Lingam).

But what about stars more massive than the Sun? How habitable are their planets? This is the question addressed in the new paper.

Massive stars dominate the light output of young stellar populations. This could have been naively interpreted to mean that they are a blessing for life. The habitable zone is farther away from these bright furnaces, potentially containing more planets. However, our paper shows that the strong winds and Ultraviolet emission extend temperate climates on habitable-zone planets to wide orbits where atmospheric retention is difficult.

The new paper couples evolutionary tracks of massive stars to climate boundaries and to atmospheric-retention limits, and characterized habitability in three complementary ways: (1) the total time a habitable-zone exists; (2) the longest continuous residence time of a planet in that zone at a fixed orbit; and (3) the maximum number of Earth analogs that can fit inside the habitable zone. These are folded results through the distribution of stellar masses in the Milky-Way galaxy to estimate the inventory of Earth analogs that satisfy the adopted climate and atmosphere-retention filters.

Our calculation shows a sharp loss of habitability for stars above 10 solar masses. At a stellar mass of 9 solar masses, habitability persists for 31 million years with characteristic radii that are 74 to 127 times larger than the Earth-Sun separation (AU), whereas at 12 solar masses — habitability becomes a brief and extremely narrow episode, lasting for just a million years at orbital radii between 256 and 263 AU. Beyond a stellar mass of 15 solar masses, there is no habitability possible. Altogether, the Milky-Way inventory of habitable planets is set by lower-mass host stars. Massive stars above 8 solar masses contribute only 0.01% of the total population of habitable-zone planets. In addition, the short lifetime of the habitable zone makes the evolution of complex forms of life less likely around massive stars.

On top of the reduced population of habitable planets near massive stars, it is difficult to detect these planets. Their wide orbits make it easier to separate them from their host stars, but the starlight reflected from their surface or their thermal emission amount to a tiny fraction of the total light emitted by the bright star — making their detection impractical even with the most ambitious coronagraph systems, designed to block the starlight.

In conclusion, we should be grateful to the Sun for its stable heat supply over billions of years. There is nothing better than a stable partner who keeps you warm and maintains the right distance from you so as not to trigger any harmful effects from its high-energy output.

Ecclesiastes 1:9 states:

What has been will be again,
what has been done will be done again;
there is nothing new under the sun.

Based on the new paper, these qualities of the Sun are a blessing. The statement “there is nothing new under the sun” will stay valid for the next billion years, before the Sun will brighten up and boil off all liquid water on Earth.

Within 7.6 billion years, the envelope of the Sun might swallow the Earth (as calculated here). Nothing good lasts forever, but while habitability lasts — lets all have fun under the Sun and stop fighting with each other.

ABOUT THE AUTHOR

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(Image Credit: Chris Michel, National Academy of Sciences, 2023)

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.

https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb

https://open.spotify.com/show/1zhndXkvSY2b8FdjspFpCd
https://x.com/ProfAviLoeb

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