This week, NASA made a huge announcement with implications for life on Mars: liquid water still flows on the surface of the red planet. Stunning images were released from reconstructions of data from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter, showing dark narrow streaks that grow and vanish seasonally. These streaks are called “recurring slope lineae” which are made of hydrated salts that confirm that liquid water flows on the surface of Mars during warmer seasons.
An impact crater on Mars with possible recurring slope lineae, a sign of flowing liquid water
Image: NASA/JPL/University of Arizona
Combine this with evidence that Mars may have once been home to a primitive ocean covering half of its surface, and the case for possible Martian life starts to look quite compelling. However, the environment on Mars is not so friendly for living creatures. Mars’s atmosphere is 100 times thinner than the Earth’s, and temperatures are a frigid average -60 degrees Celsius.
What makes a planet habitable?
Liquid water is essential to life, and is one of the defining characteristics of the habitable zone around a star: a region warm enough for water not to turn to ice, but cool enough not to boil to steam. Even on Earth, there are huge areas that are almost devoid of life, including its dry deserts and frozen polar regions. Over time, habitability on Earth has also changed. Around 300-350 million years ago, the Carboniferous period saw an environment that was warmer, wetter, and more oxygen-rich than present day Earth, allowing life to flourish both in the sea and on land. And in 5 billion years from now, the sun, having burned through most of its hydrogen fuel, will become brighter and hotter as it begins to fuse helium instead, pushing the Earth out of the habitable zone as our water evaporates away.
In fact, the Earth is just on the edge of our sun’s habitable zone. As we search for planets outside our solar system (also known as exoplanets) we often look for planets that are similar to the Earth, because it’s the only planet we know sustains life.
But are we looking in the wrong places? What would a planet look like if we were looking for one that was the most likely to support life? These so-called superhabitable planets, an idea brought forward by post doctoral fellow Rene Heller at McMaster University, may be even more liveable than the Earth.
A superhabitable planet would orbit a long-lived star
Artist’s concept of a gas giant orbiting a red K dwarf star
Image: NASA,ESA and G. Bacon (STScI)
Our own sun is 4.6 billion years old, putting it around halfway through its estimated lifespan. This steady glow of light and energy has given life lots of time to evolve on Earth. A long-lived steady star is the most important ingredient for superhabitability. However, there are stars that would make an even better candidate for a superhabitable solar system: slightly smaller K dwarf stars would have dimmer and less energetic light, but many are already billions of years older than our sun, and will continue to shine long after our sun has burned out.
Given so much more time for life to evolve, an exoplanet orbiting a K dwarf may support even more complex life, as living things modified their own environment in positive ways. For instance, here on Earth, oceanic algae are believed to have produced enough oxygen 2.4 billion years ago that the Earth’s atmosphere changed substantially, paving the way for more oxygen-intensive metabolism, and larger and more complex organisms.
A K dwarf superhabitable planet would be twice as massive as the Earth
Artist’s concept of Kepler 186f, an exoplanet in a habitable zone
Image: NASA Ames/JPL-Caltech/T. Pyle
The centre of the Earth hosts a molten core of rock, kept hot by the heat of its formation. This is important because the internal heat of the Earth is the source of volcanic activity and tectonic plate movement that keeps CO2 circulating. Without this source of CO2 for its atmosphere, a planet would plunge into freezing temperatures as all the CO2 washed out of the air during rainfall.
Also, the molten core of a rocky exoplanet like the Earth is the source of its magnetic field, and the reason why compasses point to the magnetic north. Without a magnetic field acting as a shield, the Earth’s surface would be bombarded with cosmic radiation that would be harmful to life.
A superhabitable planet orbiting in a K dwarf’s habitable zone is likely to be rocky, and would need to be twice the size of Earth to support a molten core. A planet much larger than this would be too thick for the outward flow of heat from the core to drive geological activity. A planet like this would also have a larger surface for life to thrive.
A superhabitable planet would be a flat island world
A flatter island world would encourage biodiversity
With its larger size, a superhabitable planet would have higher gravity, and a more dense atmosphere. This would give the planet thicker air and a flatter surface, as mountains would erode more quickly. The oceans on a flatter planet would also form shallower pools with many islands, instead of deep oceans with large continents. This island world may have many advantages for life, as shallow coastal waters on Earth tend to have the most biodiversity.
A superhabitable planet would have seasons
Seasons help balance out extreme warm and cold
The Earth rotates on a 23.4 degree axis which is what gives the Earth seasons. This smooths out the extreme temperature differences between the equator and the poles, and gives a larger habitable area. A superhabitable planet may even have warm, ice-free poles.