Astronomers have long known that our warm and yellow sun is a relative rarity in our galaxy, with the most common stars in the Milky Way being considerably smaller and cooler, and sporting just half of the mass of our sun at most. Although billions of planets orbit these common dwarf stars, in order to be warm enough to be habitable, they would need to huddle very close to their small stars, leaving them susceptible to extreme tidal forces.
Now, by using data collected by the Kepler and Gaia telescopes, which capture information about exoplanets as they move in front of their host stars, a team of astronomers led by the University of Florida has found that two-thirds of the planets orbiting these ubiquitous small stars are probably roasted by these tidal forces, leaving them uninhabitable.
However, one third of them – meaning hundreds of millions across the entire galaxy – could be in a so-called “goldilocks orbit” that is close enough and gentle enough to hold onto liquid water and thus possibly harbor life.
“I think this result is really important for the next decade of exoplanet research, because eyes are shifting toward this population of stars,” said study lead author Sheila Sagear, a doctoral student in Astronomy at UF. “These stars are excellent targets to look for small planets in an orbit where it’s conceivable that water might be liquid and therefore the planet might be habitable.”
The experts measured the eccentricity of over 150 planets around these M dwarf stars, which are about the size of Jupiter. The greater the oval shape of an orbit, the higher its eccentricity.
When a planet orbits in close proximity to its star – similar to Mercury’s distance from the sun – an eccentric orbit can lead to a phenomenon called tidal heating. Due to the varying gravitational forces acting on the planet as a result of its irregular orbit, it experiences stretching and deformation, which generates friction and subsequently raises its temperature. In the most extreme cases, this heating process can result in the planet being completely “baked,” eliminating any possibility of liquid water.
“It’s only for these small stars that the zone of habitability is close enough for these tidal forces to be relevant,” explained senior author Sarah Ballard, a professor of Astronomy at UF.
To measure the planets’ orbits, Ballard and Sagear used data collected by the Kepler telescope, focusing particularly on how long the planets took to move across the face of the stars. In addition, they also relied on new data from the Gaia telescope that measured the distance to billions of stars in our galaxy. “The distance is really the key piece of information we were missing before that allows us to do this analysis now,” Sagear said.
The investigation revealed that stars with multiple planets were the most likely to have the type of circular orbits allowing them to retain liquid water. By contrast, stars with only one planet were more often experiencing tidal extremes sterilizing the surface of the planet.
Since one third of the planets in this small sample had gentle enough orbits to potentially host liquid water, our galaxy may contain hundreds of millions of planets where life could develop.
The study is published in the journal Proceedings of the National Academy of Sciences.
A habitable planet is one that has conditions that are “just right” for life as we understand it. However, the exact conditions required for a planet to be habitable can depend on a variety of factors, some of which we may not yet fully understand.
Generally, though, scientists consider a few main factors when considering if a planet is potentially habitable:
The planet needs to be at the right distance from its star where it’s not too hot and not too cold. This region is often called the “habitable zone” or the “Goldilocks zone.” If a planet is in its star’s habitable zone, it could potentially have liquid water on its surface, a key ingredient for life as we know it.
Liquid water is essential for life as we know it. Therefore, for a planet to be considered habitable, there must be a likelihood of the presence of water. This means the planet should not only be in the habitable zone of its star but also have the right atmospheric pressure to maintain water in liquid form.
A planet needs a stable and suitable atmosphere for life to exist. The atmosphere plays a role in maintaining the right temperature range and pressure for liquid water. It also helps protect the planet (and any potential life) from harmful radiation from space.
The planet needs to be of the right size and composition. It should be rocky (like Earth), and have a sufficient mass to maintain its atmosphere. Planets that are too small (like Mars) can lose their atmospheres over time.
Geological activity such as volcanism and tectonic activity can play a role in maintaining a planet’s habitability over long timescales, by cycling important elements and maintaining a planet’s magnetic field.
A planet with a stable orbit and rotation is more likely to have a stable climate. A stable rotation could prevent temperature extremes between day and night.
A strong magnetic field can protect a planet’s atmosphere from being stripped away by solar wind, and shield the surface from harmful cosmic and solar radiation.
It is important to note that these are the conditions necessary for life as we know it. There could be other forms of life that have adapted to conditions we would find inhospitable.