The holy grail of planetary science is the discovery of life beyond Earth. Life on Earth requires liquid water, so scientists define a star’s habitable zone as the region of space where liquid water can exist on a planet’s surface. Of course, there are other habitability factors to consider. For example, planets with a similar size and mass to Earth might also have a similar atmosphere that supports life.

A team of international scientists recently used the TESS space telescope to discover two exoplanets – planets outside of our solar system – in the habitable zone of a small, red star called TOI-715. The larger of the two planets is classified as a “super-Earth,” as it is about 1.5 times larger than Earth. While the evidence for the second planet is tentative, it appears to be the same size as Earth, which would make it the smallest known habitable-zone exoplanet. Though both planets fit comfortably inside the habitable zone, they have some stark contrasts to Earth. Since TOI-715 is much cooler than the Sun, its habitable zone is closer to the star. Consequently, the larger and smaller exoplanets complete one orbit every 19 days and 26 days respectively. Those are some short years! Even stranger, planets around small stars like TOI-715 are susceptible to a phenomenon called tidal locking. Just as one side of the moon always faces Earth, tidally-locked planets only ever show one face to their stars. Thus, they have permanent day and night sides. This would give rise to extreme temperatures, possibly rendering the planets uninhabitable.

Habitable-zone planets are more difficult to discover than those closer to their stars. Most detection methods rely on subtle behaviors imparted on a star by an orbiting planet, and those effects become even more subtle for planets farther away. The recent additions to the small but growing list of habitable-zone planets are exciting for astronomers. Perhaps they take us a step closer to answering the age-old question: are we alone?

This study was led by Georgina Dransfield, a postdoctoral researcher at the University of Birmingham.

Managing Correspondent: Collin Cherubim

Press Article: Discovery Alert: A ‘Super-Earth’ in the Habitable Zone (NASA)

Original Journal Article: A 1.55 R_⊕ habitable-zone planet hosted by TOI-715, an M4 star near the ecliptic South Pole (Monthly Notices of the Royal Astronomical Society)

Image Credit: AdisResic/Pixabay