Astronomers should focus on systems with 'K' stars to find habitable planets, study suggests

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An artist's concept of a planet orbiting in the habitable zone of a K star. Image: NASA Ames/JPL-Caltech/Tim Pyle
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An artist's concept of a planet orbiting in the habitable zone of a K star. Image: NASA Ames/JPL-Caltech/Tim Pyle

'K' stars last longer than stars like the Sun, providing plenty of time for life to evolve on orbiting planets

Scientists have identified a class of stars that, they suggest, are most likely to have habitable planets orbiting them and could therefore be promising targets to search for signs of life in the Universe.

Scientists have named the class of stars as 'K' stars. According to them, these stars are much dimmer than Sun-like stars but brighter than the faintest stars (M-stars/red dwarfs).

Traditionally, astronomers have focused on M stars in their search for habitable planets in the universe. K stars stars account for nearly 75 per cent of all the stars in the universe and can last for more than a trillion years. They also consume their fuel in a more consistent and economical manner.

While searching for potential habitable planets, scientists try to observe the simultaneous presence of methane and oxygen in a planet's atmosphere

But there is a problem that hinders M-stars from allowing life to potentially evolve on surrounding planets.

During their youth, they are much brighter and exhibit intense stellar flares (emissions of magnetic energy). This phase continues for about one billion years after the formation of the star, and the energy released during this period is more than enough to evaporate oceans on nearby planets.

K-stars come with some unique features. They exhibit less extreme activity compared to M-stars during their early phases. They also last much longer than the Sun, providing plenty of time for life to evolve on nearby planets.

K stars typically shine for between 17 billion to 70 billion years, while Sun-like stars are estimated to last for around 10 billion years.

"I like to think that K stars are in a 'sweet spot' between Sun-analogue stars and M stars," said Giada Arney, the lead researcher from NASA's Goddard Space Flight Centre in Greenbelt, Maryland.

In the current study, Arney focused on bio-signatures that a hypothetical planet orbiting a K star would exhibit.

While searching for potential habitable planets, scientists try to observe the simultaneous presence of methane and oxygen in a planet's atmosphere. These gases quickly react with each other to produce new compounds. If they are present in an atmosphere simultaneously, it would suggest something on the planet is producing them quickly, possibly some form of life.

Using a computer model, Arney simulated the temperature and chemistry of a planetary atmosphere to determine the characteristics of that atmosphere in different host stars.

The analysis revealed that the bio-signature of methane and oxygen exhibited by planetary atmosphere would be strongest when the planet orbited a K star.

According to Arney, it happens as the oxygen doesn't destroy methane very quickly, so more of these gases build up in the atmosphere.

Arney's study also discusses a number of K stars that could provide promising targets for future observations. These are Epsilon Indi, 61 Cyg A/B, HD 156026, and Groombridge 1618.

The detailed findings of the study are published in the Astrophysical Journal Letters.

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