Investigating how stellar threats affect the habitable zone of exoplanets

This image from the study shows the sky positions of exoplanet-dominated stars projected onto the Mollweide map. The HZS is indicated by a yellow-green circle, and the remaining exoplanet population is represented by a gray circle. The studied sample of 84 HZS located within 220 parsecs of the Sun is represented by a yellow-green cross circle. The three dense HZSs located near the galactic plane are labeled 1, 2, and 3 in white. The color bar represents star density, or the number of stars with more than 15 stars within a 5-arcmin radius. Credit: arXiv (2024). DOI: 10.48550/arxiv.2410.22396
When we think about exoplanets that have the potential to support life, we focus on the habitable zone. The habitable zone is the region around a star that receives enough stellar energy for the planet to have liquid surface water. This is a somewhat crude but useful first step when looking at thousands of exoplanets.
But there’s more to livability than that.
In a dense stellar environment, planets in the habitable zone have other things to contend with besides their host star. An approaching star or supernova explosion can eject a habitable zone exoplanet from the solar system, destroying its atmosphere and even the planet itself.
A new study investigates the threats facing planets in the habitable zone of our star’s neighborhood. The study, titled “Ten Neighborhoods of Habitable Zone Exoplanet Systems: Threat Assessment from Stellar Encounters and Supernovae,” has been accepted for publication in The Astronomical Journal. Currently displayed on the arXiv preprint server. The lead author is Tishagupta Pine of the Center for Integrated Science Education and Research, Visva Bharati University, India.
The researchers surveyed 10 parsec regions around 84 solar systems with habitable zone exoplanets. Some of these habitable zone systems (HZS) face risks from stars outside our solar system. How do these risks affect habitability? What does it mean for our concept of the habitable zone?
“Of the more than 4,500 exoplanet-hosted stars, approximately 140 or more are known to host planets within their habitable zones,” the authors write. “We are assessing the risks that the local stellar environments of these ultrahot zones may pose to their habitability.”
More than 150 exoplanets have been identified in the habitable zone, and as exoplanet science advances, scientists are developing a more detailed understanding of what the “habitable zone” means. . Scientists are increasingly using the terms “conservative habitable zone” and “optimistic habitable zone.”
The optimistic habitable zone is defined as a region that receives less radiation from its star than Venus received 1 billion years ago and more than Mars received 3.8 billion years ago. Scientists believe that recent Venus (RV) and early Mars (EM) both likely had surface water.
A conservative habitable zone is a stricter definition. This is a narrow region around a star where an exoplanet may have surface water. It is defined by an inner runaway greenhouse edge where the stellar flux evaporates surface water and an outer maximum greenhouse edge where the carbon dioxide greenhouse effect is dominated by Rayleigh scattering.
These are useful scientific definitions insofar. But what about the environments of habitable stars? In recent years, scientists have learned a lot about how stars behave, the characteristics of exoplanets, and how they intertwine. I did.
“The discovery of a large number of exoplanets has revealed a variety of characteristics of stars and planets, and systematic comparisons are important to assess their habitability and the possibility that life exists outside our solar system.” ,” the authors write.
To make these necessary systematic comparisons, researchers developed two metrics: the Solar Similarity Index (SSI) and the Neighborhood Similarity Index (NSI). Main-sequence stars like our Sun facilitate habitability, so SSI compares the properties of our solar system to those of other HZs. NSI compares the properties of stars in a 10 parsec region around the Sun with regions of the same size around other HZSs.
These indicators place the habitable zone in a larger context.
“While the HZ concept is essential in the search for habitable worlds, the planet’s stellar environment also plays an important role in determining its lifetime and maintenance of habitability,” the authors write. “Studies have shown that frequent catastrophic events such as supernovae and stellar approaches in regions of high stellar density do not support the evolution of complex life forms or the maintenance of long-term habitability. ”
When radiation and high-energy particles from distant sources reach planets in their habitable zones, they can seriously damage Earth-like planets. A supernova is a source of dangerous radiation and particles, and if it exploded near Earth, it would be the end of life. Scientists know that ancient supernovae left traces on Earth, but none of them came close enough to destroy the atmosphere.
“Our main focus is to investigate the influence of SNe on the atmosphere of an exoplanet or exomoon, assuming that the atmosphere is similar to Earth,” the authors write.
The first factor is the density of the star. The more stars there are nearby, the more likely there will be a supernova explosion or a close stellar encounter.
“The astrophysical influence of the stellar environment is a ‘low-probability, high-consequence’ scenario for the continued habitability of exoplanets,” the authors write. Although catastrophic events such as supernova explosions or close stellar encounters are unlikely, their effects could be so severe that they could completely eliminate habitability.
As for the threat of supernovae, only massive stars explode, so the researchers focused on massive stars in the star’s neighborhood. Pine and colleagues discovered two high-mass stars with more than eight solar masses in the 10-parsec neighborhood of HZS, TOI-1227 and HD 48265.
“These high-mass stars are potential precursors to supernova explosions,” the authors explain.
Only one of the HZS is at risk of a stellar flyby. The encounter rate for HD 165155 is at least once every 5 Gyr period. That means there’s a greater risk of encountering another star that could eject the planet from its habitable zone.
The researchers’ pair of indices, SSI and NSI, yielded different results. “…we found that the stellar environment of most HZSs shows a high degree of similarity (NSI>0.75) to the solar neighborhood,” the researchers explain. However, the HZS has a wide variety of stars, so it has a wider range of SSI values compared to the Sun.
We know the dangers supernova explosions pose to habitability. The initial burst of radiation could kill anything too close to the planet’s surface. The ongoing radiation could strip away the atmospheres of some distant planets, and could also cause DNA damage in any life forms exposed to it.
For planets far away from the explosion, supernovae can change the climate and cause extinctions. Although there is no completely certain understanding of how far away a planet would have to be to avoid devastation, many scientists say that within 50 light years a planet would probably be scorched.
You can see the results of some of the stellar flybys that the authors are considering. Rogue planets, or free-floating planets (FPPs), may be in an unfortunate situation because a stellar invader gets too close to the star, disrupting the gravitational relationship between the planet and the star.
We don’t know how many of these FPPs there are in the Milky Way, but there could be billions. Future telescopes like the Nancy Grace Roman Space Telescope will help us understand how many there really are.
Habitability may be temporary, and our planet may be an exception. Although life appears in the habitable zone of many planets, it may not persist for long due to various factors. From a great distance, it is not possible to detect all the variables involved in an exoplanet’s habitability.
But it is possible to understand the environments of stars that host potentially habitable exoplanets, and this study shows how.
Further information: Tisyagupta Pyne et al. “Ten Neighborhoods of Habitable Zone Exoplanet Systems: Threat Assessment from Stellar Encounters and Supernovae”, arXiv (2024). DOI: 10.48550/arxiv.2410.22396
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