A nearby exoplanet could maintain conditions allowing water to remain liquid despite major variations in the energy it receives over the course of its year.
Gl 514 b is located about 25 light-years from Earth, orbiting an M0.5-type red dwarf star. Its distance from its star places it partially within the so-called habitable zone. In this region, water could theoretically remain liquid on a planet's surface if its atmosphere and other properties are suitable.
While Earth and the other planets in our solar system orbit the Sun in nearly circular orbits, planets in other systems may have more comet-like orbits, where the distance between the planet and its star varies. These orbits, known as eccentric orbits, cause the planet to enter and leave the habitable zone.
The habitable zone, shown in green here, is defined as the region around a star where liquid water, an essential ingredient for life as we know it, could potentially exist.
Earth, by comparison, remains within its habitable zone at all times.
NASA image
Why only partially within the habitable zone? Because Gl 514 b follows a highly elongated orbit: its eccentricity is estimated at around 0.45, though there is still considerable uncertainty. In other words, its distance from the star varies greatly during one revolution. The amount of energy it receives therefore changes as well, which could cause major seasonal variations.
The researchers simulated numerous possible climates rather than settling on a single scenario. Their model takes into account the orbit, the tilt of the rotation axis and the amount of atmospheric carbon dioxide. The proportion of exposed land and its distribution were also varied between simulations.
Two states appear most often. In some scenarios, the planet becomes almost entirely covered in ice. In others, its surface remains largely ice-free. Only 1.27% of the simulations produce an intermediate situation, with polar ice caps.
Carbon dioxide then plays a decisive role, because this gas retains some of the heat. The simulations indicate that a CO₂ partial pressure between 7.25 and 9.5 bars could produce temperatures compatible with liquid water on the surface. This pressure is several times the total atmospheric pressure measured at sea level on Earth.
The researchers are exploring possible conditions based on the available data. The study therefore detects neither an ocean nor a CO₂-rich atmosphere. Instead, it determines which properties would be necessary to make certain regions of the surface habitable.
Ice is also of interest for future observations. A bright surface does not reflect light in the same way as a dark surface. The amount and location of frozen regions could therefore alter the light signal measured when telescopes attempt to directly observe this planet.
Gl 514 b is close enough to interest future instruments capable of separating a planet's faint light from that of its star. The simulations now provide several reference climates to compare with these observations. They could help determine whether future data are more consistent with a frozen, temperate or nearly ice-free planet.