Scientists have changed the rules for what constitutes the habitable zone for exoplanets — a Goldilocks-like range of not-too-close, not-too-far distances from the parent star that might allow the planet to host life.
The redefined habitable zone boundaries mean that some planets that were once considered potentially habitable have been kicked out, while others that were once excluded may be reexamined and are now considered potentially habitable.
Ravi Kumar Kopparapu, who works at Penn State University and led the research team, believes the previously included exoplanets will take the biggest hit from the new habitable zone rules.
“This will have a significant impact on the number of exoplanets that are within habitable zone,” he said.
According to Abel Méndez, who works for the University of Puerto Rico at Arecibo, previously excluded planets may also be affected.
“Many of those planets that we believe were inside are now outside,” said Méndez, who was not involved in the research. “But on the other side, it extends the habitable zone’s outer edge, so a few planets that are farther away might fall inside the habitable zone now.”
The main goal of the habitable zone is to determine which planets would be able to hold water in its liquid form on the surface. Being closer to the parent star would cause the water to evaporate into the air, while planets that lie farther away would cause it to freeze to ice.
Now, the habitable zone will also be determined by two updated atmospheric databases: the high-resolution transmission molecular absorption (HITRAN) and the high-temperature spectroscopic absorption parameters (HITEMP). These two databases find the absorption parameters of water and carbon dioxide. Water and carbon dioxide are crucial to atmosphere of exoplanets and can help scientists determine if the exoplanet could hold liquid water.
Despite making changes that will result in better predictions of habitability, the habitable zone is not perfect. According to the researchers, the model does not factor in feedback from clouds on the planets. This feedback could affect whether the planet could ever hold life.
Twenty year ago, James Kasting of Penn State University developed the previous habitable zone guidelines. Kasting was also part of the new updates to the habitable zone definition.
According to Kopparapu, Kasting’s initial work has led to the discovery of a lot of new planets. “At the time when he wrote that paper no exoplanets were discovered,” he told SPACE.com. “In 20 years, hundreds, maybe thousands have been discovered.”