A team of geobiologists and astronomers from the United States and Germany has identified 24 extrasolar planets that may have conditions more suitable for life than Earth. Those alien worlds are older, a little larger, slightly warmer and possibly wetter than our own planet.
“The 24 top contenders for superhabitable planets are all more than 100 light-years away, but our study could help focus future observation efforts, such as from NASA’s James Web Space Telescope, NASA’s LUVIOR (Large UV/Optical/IR Surveyor) space observatory and ESA’s PLATO (PLAnetary Transits and Oscillations of stars) space telescope,” said Professor Dirk Schulze-Makuch, a geobiologist at Washington State University and the Technical University in Berlin.
“With the next space telescopes coming up, we will get more information, so it is important to select some targets.”
“We have to focus on certain planets that have the most promising conditions for complex life. However, we have to be careful to not get stuck looking for a second Earth because there could be planets that might be more suitable for life than ours.”
In the study, Professor Schulze-Makuch and his colleagues, Dr. Rene Heller from the Max Planck Institute for Solar System Research and Dr. Edward Guinan from Villanova University, identified superhabitability criteria and searched among the 4,500 known exoplanets for good candidates.
They selected stellar systems with probable terrestrial planets orbiting within the host star’s liquid water habitable zone from the Kepler Object of Interest Exoplanet Archive of transiting exoplanets.
“While the Sun is the center of our Solar System, it has a relatively short lifespan of less than 10 billion years,” they said.
“Since it took nearly 4 billion years before any form of complex life appeared on Earth, many similar stars to our Sun, called G-type stars, might run out of fuel before complex life can develop.”
“In addition to looking at systems with cooler G-type stars, we also looked at systems with K-dwarf stars, which are somewhat cooler, less massive and less luminous than our Sun. K stars have the advantage of long lifespans of 20 to 70 billion years.”
“This would allow orbiting planets to be older as well as giving life more time to advance to the complexity currently found on Earth.”
Star-planet distances (along the abscissa) and mass of the host star (along the ordinate) of roughly 4,500 confirmed and candidate exoplanets. The temperatures of the stars are indicated with symbol colors (see color bar). Planetary radii are encoded in the symbol sizes (see size scale at the bottom). The conservative habitable zone, defined by the moist-greenhouse and the maximum greenhouse limits, is outlined with black solid lines. Stellar luminosities required for the parameterization of these limits are shown along the ordinate of the diagram. The dashed box refers to the region shown in the next figure. Image credit: Schulze-Makuch et al, doi: 10.1089/ast.2019.2161.
However, to be habitable, planets should not be so old that they have exhausted their geothermal heat and lack protective geomagnetic fields.
Earth is around 4.5 billion years old, but the scientists argue that the sweet spot for life is a planet that is between 5 billion to 8 billion years old.
“Size and mass also matter. A planet that is 10% larger than the Earth should have more habitable land,” they said.
“One that is about 1.5 times Earth’s mass would be expected to retain its interior heating through radioactive decay longer and would also have a stronger gravity to retain an atmosphere over a longer time period.”
“Water is key to life and a little more of it would help, especially in the form of moisture, clouds and humidity.”
The habitable zone around K-dwarf stars, the potential site for superhabitable planets. Twenty-four confirmed and candidate exoplanets that are smaller than 2 Earth radii are labeled with name tags. Uncertainties in the observed stellar, planetary, and orbital parameters propagate into the planetary radius measurements, which is why we include planets as large as 2 Earth radii, although truly superhabitable planets might be restricted to radii less than 1.1 Earth radii. Image credit: Schulze-Makuch et al, doi: 10.1089/ast.2019.2161.
A slightly overall warmer temperature, a mean surface temperature of about 5 degrees Celsius (or about 8 degrees Fahrenheit) greater than Earth, together with the additional moisture, would be also better for life.
This warmth and moisture preference is seen on Earth with the greater biodiversity in tropical rain forests than in colder, drier areas.
Among the 24 top planet candidates none of them meet all the criteria for superhabitable planets, but one has four of the critical characteristics, making it possibly much more comfortable for life than our home planet.
“It’s sometimes difficult to convey this principle of superhabitable planets because we think we have the best planet,” Professor Schulze-Makuch said.
“We have a great number of complex and diverse lifeforms, and many that can survive in extreme environments. It is good to have adaptable life, but that doesn’t mean that we have the best of everything.”
The team’s paper was published in the journal Astrobiology.
Dirk Schulze-Makuch et al. In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World. Astrobiology, published online September 18, 2020; doi: 10.1089/ast.2019.2161
This article is based on a press-release provided by Washington State University.