A category of small, orange stars may be the best place to look for extra-terrestrial life. So called K-type stars, which are smaller and cooler than the Sun, could provide the perfect conditions for exoplanets to host life, according to a new study by NASA space scientist Dr Giada Arney.
Since K-type stars are relatively cool, they burn through their fuel slowly and survive for a long time. While hotter stars like our Sun, which is classified as a G2V-type, yellow dwarf star, will only survive for 10 billion years, K-type stars will shine for at least 15 billion years, possibly even up to 70 billion. During this time, they exhibit very little extreme activity that could disrupt evolving life.
Red dwarfs, also known as M-type stars, are a common target in the search for habitable exoplanets. However, these stars go through a violent phase in their youth that can be inhospitable to life. They throw tantrums in the form of stellar flares, and can be hot enough to boil away water.
On the other hand, K-type stars produce few stellar flares and their temperature stays steady throughout their lifetime. “I like to think that K stars are in a ‘sweet spot’ between Sun-analog stars and M stars,” said Arney.
Arney suggests that exoplanets around K-type stars could be the best place to look for signs of life, or biosignatures. In particular, high levels of methane and oxygen in the atmosphere are considered a biosignature since oxygen and methane naturally react and destroy each other. So, if an atmosphere has high levels of the gases, there must be processes to produce them.
However, exoplanets are so far away from Earth that biosignatures must be strong to be detected. The energetic oxygen gases needed to destroy methane are produced by ultraviolet light. “When you put the planet around a K star, the oxygen does not destroy the methane as rapidly, so more of it can build up in the atmosphere,” said Arney.