When we think of exoplanets that may be able to support life, we hone in on the habitable zone. A habitable zone is a region around a star where planets receive enough stellar energy to have liquid surface water. It’s a somewhat crude but helpful first step when examining thousands of exoplanets.
However, there’s a lot more to habitability than that.
In a dense stellar environment, planets in habitable zones have more than their host star to contend with. Stellar flybys and exploding supernovae can eject habitable zone exoplanets from their solar systems and even destroy their atmospheres or the planets themselves.
New research examines the threats facing the habitable zone planets in our stellar neighbourhood. The study is “The 10 pc Neighborhood of Habitable Zone Exoplanetary Systems: Threat Assessment from Stellar Encounters & Supernovae,” and it has been accepted for publication in The Astronomical Journal. The lead author is Tisyagupta Pyne from the Integrated Science Education And Research Centre at Visva-Bharati University in India.
The researchers examined the 10-parsec regions around the 84 solar systems with habitable zone exoplanets. Some of these Habitable Zone Systems (HZS) face risks from stars outside of the solar systems. How do these risks affect their habitability? What does it mean for our notion of the habitable zone?
“Among the 4,500+ exoplanet-hosting stars, about 140+ are known to host planets in their habitable zones,” the authors write. “We assess the possible risks that local stellar environment of these HZS pose to their habitability.”
This image from the research shows the sky positions of exoplanet-hosting stars projected on a Molleweide map. HZS are denoted by yellow-green circles, while the remaining population of exoplanets is represented by gray circles. The studied sample of 84 HZS, located within 220 pc of the Sun, is represented by crossed yellow-green circles. The three high-density HZS located near the galactic plane are labeled 1, 2 and 3 in white. The colour bar represents the stellar density, i.e., the number of stars having more than 15 stars within a radius of 5 arc mins. Image Credit: Pyne et al. 2024. We have more than 150 confirmed exoplanets in habitable zones, and as exoplanet science advances, scientists are developing a more detailed understanding of what habitable zone means. Scientists increasingly use the terms conservative habitable zone and optimistic habitable zone.
The optimistic habitable zone is defined as regions that receive less radiation from their star than Venus received one billion years ago and more than Mars did 3.8 billion years ago. Scientists think that recent Venus (RV) and early Mars (EM) both likely had surface water.
The conservative habitable zone is a more stringent definition. It’s a narrower region around a star where an exoplanet could have surface water. It’s defined by an inner runaway greenhouse edge where stellar flux would vaporize surface water and an outer maximum greenhouse edge where the greenhouse effect of carbon dioxide is dominated by Rayleigh scattering.
Those are useful scientific definitions as far as they go. But what about habitable stellar environments? In recent years, scientists have learned a lot about how stars behave, the characteristics of exoplanets, and how the two are intertwined.
“The discovery of numerous extrasolar planets has revealed a diverse array of stellar and planetary characteristics, making systematic comparisons crucial for evaluating habitability and assessing the potential for life beyond our solar system,” the authors write.
To make these necessary systematic comparisons, the researchers developed two metrics: the Solar Similarity Index (SSI) and the Neighborhood Similarity Index (NSI). Since main sequence stars like our Sun are conducive to habitability, the SSI compares our Solar System’s properties with those of other HZs. The NSI compares the properties of stars in a 10-parsec region around the Sun to the same size region around other HZSs.
This research is mostly based on data from the ESA’s Gaia spacecraft, which is building a map of the Milky Way by measuring one billion stars. But the further away an HZS is, or the dimmer the stars are, the more likely Gaia may not have detected every star, which affects the research’s results. This image shows Gaia’s data completeness. The colour scale indicates the faintest G magnitude at which the 95% completeness threshold is achieved. “Our sample of 84 HZS (green circles) has been overlaid on the map to visually depict the completeness of their respective neighbourhoods,” the authors write. Image Credit: Pyne et al. 2024. These indices put habitable zones in a larger context.
“While the concept of HZ is vital in the search for habitable worlds, the stellar environment of the planet also plays an important role in determining longevity and maintenance of habitability,” the authors write. “Studies have shown that a high rate of catastrophic events, such as supernovae and close stellar encounters in regions of high stellar density, is not conducive to the evolution of complex life forms and the maintenance of habitability over long periods.”
When radiation and high-energy particles from a distant source reach a planet in a habitable zone, they can cause severe damage to Earth-like planets. Supernovae are a dangerous source of radiation and particles, and if one were to explode close enough to Earth, that would be the end of life. Scientists know that ancient supernovae have left their mark on Earth, but none of them were close enough to destroy the atmosphere.
“Our primary focus is to investigate the effects of SNe on the atmospheres of exoplanets or exomoons assuming their atmospheres to be Earth-like,” the authors write.
The first factor is stellar density. The more stars in a neighbourhood, the greater the likelihood of supernova explosions and stellar flybys.
“The astrophysical impacts of the stellar environment is a “low-probability, high-consequence” scenario
for the continuation of habitability of exoplanets,” the authors write. Though disruptive events like supernova explosions or close stellar flybys are unlikely, the consequences can be so severe that habitability is completely eliminated.
When it came to the supernova threat, the researchers looked at high-mass stars in stellar neighbourhoods since only massive stars explode. Pyne and her colleagues found high-mass stars with more than eight solar masses in the 10-parsec neighbourhoods of two HZS: TOI-1227 and HD 48265. “These high-mass stars are potential progenitors for supernova explosions,” the authors explain.
Only one of the HZS is at risk of a stellar flyby. HD 165155 has an encounter rate of ?1 in 5 Gyr period. That means it’s at greater risk of an encounter with another star that could eject planets from its habitable zone.
The team’s pair of indices, the SSI and the NSI, produced divergent results. “… we find that the stellar environments of the majority of HZS exhibit a high degree of similarity (NSI> 0.75) to the solar neighbourhood,” they explain. However, because of the wide variety of stars in HZS, comparing them to the Sun results in a wide range of SSI values.
We know the danger supernova explosions pose to habitability. The initial burst of radiation could kill anything on the surface of a planet too close. The ongoing radiation could strip away the atmospheres of some planets further away and could also cause DNA damage in any lifeforms exposed to it. For planets that are further away from the blast, the supernova could alter their climate and trigger extinctions. There’s no absolutely certain understanding of how far away a planet needs to avoid devastation, but many scientists say that within 50 light-years, a planet is probably toast.
We can see the results of some of the stellar flybys the authors are considering. Rogue planets, or free-floating planets (FPPs), are likely in their hapless situations precisely because a stellar interloper got too close to their HZS and disrupted the gravitational relationships between the planets and their stars. We don’t know how many of these FPPs are in the Milky Way, but there could be many billions of them. Future telescopes like the Nancy Grace Roman Space Telescope will help us understand how many there truly are.
An artist’s illustration of a rogue planet, dark and mysterious. Image Credit: NASAHabitability may be fleeting, and our planet may be the exception. It’s possible that life appears on many planets in habitable zones but can’t last long due to various factors. From a great distance away, we can’t detect all the variables that go into exoplanet habitability.
However, we can gain an understanding of the stellar environments in which potentially habitable exoplanets exist, and this research shows us how.
