In our search for life as we know it on planets beyond our solar system, the first thing astronomers look for is whether the planet lies in the so-called “Goldilocks zone.” This refers to the region around a star where the temperature is just right — not too hot and not too cold — for liquid water to exist.

A new study published in the journal Science Advances now details a second Goldilocks factor that needs to be taken into account while searching for habitable planets — a planet’s internal temperature during its formative years.

It was long believed that planets could self-regulate their internal temperature through a process known as “mantle convection,” wherein heat is slowly transported from the planet’s interior by underground shifting of rocks. Therefore, even if a planet started out too cold or too hot, scientists believed it would eventually settle into the right temperature.

However, the new study, carried out by Jun Korenaga — a professor of geology and geophysics at Yale University — argues that it is highly unlikely that Earth-like planets self-regulate their internal temperature through mantle convection.

“If you assemble all kinds of scientific data on how Earth has evolved in the past few billion years and try to make sense out of them, you eventually realize that mantle convection is rather indifferent to the internal temperature,” Korenaga said in a statement.

The findings of this study can help us further narrow down our search for potentially habitable exoplanets. Current estimates suggest that there may be as many as 40 billion Earth-sized planets locked in the habitable zones of sun-like or red dwarf stars in the Milky Way alone. Given that even the most conservative estimate pegs the total number of galaxies in the universe at 100 billion, the total number of Earth-sized planets in Goldilocks zones adds up to a mind-bending 4 sextillion.

NASA’s Kepler Space Telescope has so far detected over 4,700 exoplanet candidates, of which only a few dozen lie in the habitable zone. If the Goldilocks factor for planets’ internal temperatures is taken into consideration, this number is likely to be whittled down further.

“The lack of the self-regulating mechanism has enormous implications for planetary habitability,” Korenaga said in the statement. “What we take for granted on this planet, such as oceans and continents, would not exist if the internal temperature of Earth had not been in a certain range, and this means that the beginning of Earth’s history cannot be too hot or too cold.”