Earth may not be the greatest planet in the universe, according to new research. Outside our solar system, astronomers have found two dozen planets that could have environments more conducive for life than our own. Some of these stars in space could be greater than our sun, too.

A study recently published in the journal Astrobiology by Washington State University scientist Dirk Schulze-Makuch outlines the attributes of possible "superhabitable" planets, including those that are older, slightly larger, marginally warmer, and probably wetter than Earth. Life may also grow more easily on planets that orbit stars with longer lifespans that shift more slowly than our own.

The 24 top candidates for superhabitable planets are more than 100 light-years distant, but Schulze-Makuch said the research could help focus potential observation efforts, such as the James Web Space Telescope of NASA, the LUVIOR space observatory, and the PLATO Space Telescope of the European Space Agency.

"We have to focus on certain planets that have the most promising conditions for complex life," Schulze-Makuch said. "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."

For the report, a geobiologist with experience in planetary habitability, Schulze-Makuch collaborated with astronomers Rene Heller of the Max Planck Institute for Solar System Research and Edward Guinan of Villanova University to define parameters for superhabitability and check for appropriate candidates among the 4,500 identified exoplanets outside our solar system. Habitability does not mean that these planets certainly have life, only the situations that will lead to life.

From the Kepler Object of Interest Exoplanet Archive of transiting exoplanets, the researchers picked planet-star systems with possible terrestrial planets orbiting within the liquid water habitable zone of the host star.

Although the center of our solar system is the sun, its lifetime is comparatively short, less than 10 billion years. Because it took almost 4 billion years before any sort of complex life on Earth existed, many of our sun's related stars, called G stars, may run out of fuel before complex life can evolve.

Water is important for life, and the authors argue that a little more, especially in the form of moisture, clouds, and humidity, will help. It would also be safer for life to have a somewhat warmer ambient climate, an average surface temperature of about 8 degrees Fahrenheit greater than Earth, along with extra moisture. This preference for moisture and humidity is found on Earth in tropical rain forests with greater biodiversity than in cooler, drier regions.

None of the 24 top planet contenders satisfy all the requirements for superhabitable planets, but one has four essential attributes, making life much more pleasant than our home planet.