Could extraterrestrial life successfully leave Proxima b, our closest exoplanet?

Artist's impression / Credits: ESO/M. Kornmesser

A civilisation situated in the habitable area of a dwarf star such as Proxima Centauri could in fact have difficulty getting into interstellar space using conventional rockets.

Since the beginning of the space exploration era, humans have used conventional chemically propelled rockets in order to get into space and escape the Earth’s gravitational attraction. Although this method is effective, it is also very costly and requires considerable resources. While we are seeking more effective means of propulsion, certain researchers are asking themselves whether equally advanced species on other planets (where the conditions are different) could use similar methods.

On Earth, spacecrafts rely on Newton’s third law: a rocket is pushed forward by ejecting burned fuel gases backwards. For example, if you push hard against an immovable wall while you are wearing ice skates, you will be propelled backwards with the same force as you have exerted on the wall. If a rocket needs to leave the orbit of the Earth and reach space, it needs to reach a speed of 11,186 km/s (which is 40,270 km/h). This escape method depends on the composition and quantity of fuel, as well as the temperature at which it is burned.

However, this method has its limits: it is difficult to go beyond 12 km/s. This has put a speed limit on all spacecrafts launched to date by NASA and other space agencies. By happy coincidence, the speed required to leave the orbit of the Earth around the Sun is 11 kilometers per second. This miracle has allowed our civilisation to embark upon missions such as Voyager 1 and 2, or New Horizons, which have traveled far and wide throughout the solar system. But are other civilisations just as lucky?

Life as we know it requires liquid water, which could exist on planets with a similar surface temperature and with a similar mass to the Earth. Since the surface temperature of a hot planet is dictated by stellar irradiation flux, the distance of the habitable area around any star is roughly equivalent to the square root of the star’s brightness. For low mass stars, stellar brightness is calculated as the stellar mass to the power of three. The escape speed is roughly the square root of the stellar mass over the distance from the star.

All in all, these factors imply that the take-off speed from the habitable area of a star inversely correlates with the stellar mass to the power of a quarter. Paradoxically, the gravitational potential is much stronger in the habitable area around stars with inferior masses. A civilisation born near to a dwarf star – such as Proxima Centauri – would need to launch rockets at an even higher speed, in order to escape the gravitational force of their star, even if this star is smaller than the sun.

Low mass stars – red dwarves – are the most common type of stars and make up around 75% of the stellar population. At 4.5 light years from the Earth, Proxima Centauri shelters a planet called Proxima b, situated in its habitable area, at a distance 20 times shorter than the distance between the Earth and the sun. Escape velocity from Proxima b, in order to get away from its star, would thus be around 65 kilometers per second. This is bad news for any potential technological civilisations living in the habitable area of dwarf stars.

Their spatial missions would in fact be barely capable of reaching interstellar space by using only chemical propulsion. Launching a rocket in the direction the planet moves would reduce the speed required by around 30 kilometers per second. But still, this is far too much. Looking at it in this way should make us count ourselves lucky: we live in the habitable zone of a rare star. Not only do we have liquid water and a comfortable enough climate  to maintain a good quality of life, but we also live on a platform from which we can escape relatively easily into interstellar space.