“Somewhere, on some other planet orbiting some very distant star, maybe in another galaxy, there could well be entities that are at least as intelligent as we are, and are interested in science. It is not impossible,” says Nobel-Prize laureate, physicist Murray Gell-Mann. “I think there probably are lots. Very likely none is close enough to interact with us, but they could be out there very easily.”
Gell-Mann’s entities may inhabit one of the two dozen planets outside our solar system researchers have identified that may orbit stars that have created conditions more suitable for life than Earth, according to a study led by Dirk Schulze-Makuch, a professor at the Center for Astronomy and Astrophysics at the Technical University Berlin, Germany and Adjunct Professor at the School of Earth and Environmental Sciences Washington State University and author of Cosmic Biology: How Life could Evolve on Other Worlds.
The Superhabitable Planets
Schulze-Makuch’s research identifies characteristics of potential “superhabitable” planets that include those that are older, a little larger, slightly warmer and possibly wetter than Earth. Life could also more easily thrive on planets that circle more slowly changing stars with longer lifespans than our sun, perhaps, according to some conjectures, leading to advanced civilizations capable of manipulating the basic structure of space and time.
The 24 top contenders for superhabitable planets are all more than 100 light years away, but Schulze-Makuch said the study could help focus future observation efforts, such as from NASA’s James Web Space Telescope, the LUVIOR space observatory and the European Space Agency’s PLATO space telescope.
“With the next space telescopes coming up, we will get more information, so it is important to select some targets,” said Schulze-Makuch, a professor with WSU and the Technical University in Berlin. “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.”
Exist Among the4,500 Known Exoplanets
For the study, Schulze-Makuch, a geobiologist with expertise in planetary habitability teamed up with astronomers Rene Heller of the Max Planck Institute for Solar System Research and Edward Guinan of Villanova University to identify superhabitability criteria and search among the 4,500 known exoplanets beyond our solar system for good candidates. Habitability does not mean these planets definitely have life, merely the conditions that would be conducive to life.
The researchers selected planet-star 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. Since it took nearly 4 billion years before any form of complex life appeared on Earth, many similar stars to our sun, called G stars, might run out of fuel before complex life can develop.
K Star Planets –Lifespans of 70 Billion Years
In addition to looking at systems with cooler G stars, similar stars to our sun that might run out of fuel before complex life can develop.the researchers looked at systems with K dwarf stars, which are somewhat cooler, less massive and less luminous than our sun. K stars, say the Washington State team, have the advantage of long lifespans of 20 billion to 70 billion years, allowing orbiting planets to be older as well as giving life more time to advance to the complexity currently found on Earth.
Geomagnetic fields: 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 researchers argue that the sweet spot for life is a planet that is between 5 billion to 8 billion years old.
Mass. A planet that is 10% larger than the Earth should have more habitable land. 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.
Surface temperature and Water. Water is key to life and the authors argue that a little more of it would help, especially in the form of moisture, clouds and humidity. 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 suitable 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,” said Schulze-Makuch. “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.”
Source: Dirk Schulze-Makuch et al, In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World, Astrobiology (2020). DOI: 10.1089/ast.2019.2161
The Daily Galaxy, Max Goldberg, via Washington State University
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