Our blueprint for the habitability of exoplanets is just one world in the entire cosmos: Earth. The only planet on which we can be positive that life has arisen is our Earth.
But the prerequisites for life as we know it could not just exist on planets similar to Earth; in fact, researchers have recently identified one type of exoplanet that might support life for billions of years.
The secret is in liquid water that is persistent throughout time. Liquid water was essential for the origin of life here on Earth. Therefore, exoplanets with the potential to maintain liquid water may have a higher likelihood of supporting life as we know it.
Researchers have discovered that a lovely, thick atmosphere of hydrogen and helium may sustain temperatures and conditions favorable for life for a very long time, according to new study conducted by astronomer Marit Mol Lous of the University of Zürich in Switzerland.
"One of the reasons that water can be liquid on Earth is its atmosphere," according to theoretical astronomer Ravit Helled of the University of Zurich in Switzerland.
"With its natural greenhouse effect, it traps just the right amount of heat to create the right conditions for oceans, rivers, and rain."
However, the present-day appearance of Earth's atmosphere was not always the case. The main elements at this time are nitrogen and oxygen, with very small quantities of hydrogen and helium.
The Sun and the Solar System evolved from a cloud of gas and dust, and the planet originally had what is known as a primordial atmosphere, which was mostly composed of hydrogen and helium.
Early mechanisms including irradiation from a very hot young Sun and meteorite bombardment are likely to blame for Earth's loss of its original atmosphere.
A super-Earth exoplanet, which would be larger than Earth but smaller than Neptune, would be able to preserve its original atmosphere for a lot longer than Earth was able to.
"Such massive primordial atmospheres can also induce a greenhouse effect – much like Earth's atmosphere today," Helled stated. "We therefore wanted to find out if these atmospheres can help to create the necessary conditions for liquid water."
The researchers used simulations to carry out this analysis, simulating exoplanets with various core masses, atmospheric masses, and orbital distances from their host stars, which they modeled as being similar to the Sun.
Their findings demonstrated that exoplanets with a dense early atmosphere might actually be warm enough to sustain the presence of liquid water for as long as 10 billion years.
But there are restrictions. The exoplanet must be far from the star—roughly twice Earth's distance from the Sun—to avoid the powerful solar radiation that might remove a primordial atmosphere. That distance from the Sun for the Solar System makes any water on a planet's surface likely to be frozen.
The Sun isn't a planet's only source of heat, though; certain planets, like Earth, are capable of producing their own heat. Numerous mechanisms, including geothermal activities and the existence of radioactive materials that release heat during their decay, can contribute to this.
The prerequisites for liquid water at the surface would thus be satisfied if a super-Earth exoplanet at that distance from its host star had both a primordial atmosphere and enough internal heating to keep itself warm, the researchers added.
"To many, this may come as a surprise," theoretical physicist Christoph Mordasini of the University of Bern adds.
"Astronomers typically expect liquid water to occur in regions around stars that receive just the right amount of radiation: not too much, so that the water does not evaporate, and not too little, so that it does not all freeze."
"Since the availability of liquid water is a likely prerequisite for life, and life probably took many millions of years to emerge on Earth, this could greatly expand the horizon for the search for alien lifeforms. Based on our results, it could even emerge on so-called free-floating planets that do not orbit around a star."
This internal heating approach could be able to sustain life on frozen worlds like Jupiter's moon Europa and Saturn's moon Enceladus as well as moons orbiting rogue exoplanets that are floating aimlessly across the cosmos.
For the team's model, several components must be at the proper locations at the appropriate times. That's not impossible because Earth and all of its life do exist, but it might not happen quickly.
"While our results are exciting, they should be considered with a grain of salt. For such planets to have liquid water for a long time, they have to have the right amount of atmosphere. We do not know how common that is," says Mordasini.
"And even under the right conditions, it is unclear how likely it is for life to emerge in such an exotic potential habitat. That is a question for astrobiologists. Still, with our work we showed that our Earth-centered idea of a life-friendly planet might be too narrow."
The research has been published in Nature Astronomy.
