Astronomy & Astrophysics 101: Habitable Zone

TRAPPIST-1 Planetary System

This artist’s impression shows the view from the surface of one of the planets in the TRAPPIST-1 system. At least seven planets orbit this ultracool dwarf star 40 light-years from Earth and they are all roughly the same size as the Earth. Several of the planets are at the right distances from their star for liquid water to exist on the surfaces. This artist’s impression is based on the known physical parameters of the planets and stars seen, and uses a vast database of objects in the Universe. Credit: ESO/N. Bartmann/

What Is a Habitable Zone?

The Habitable Zone is the region around a star where the conditions could potentially be suitable to sustain life on a planet, for example allowing the presence of liquid water on its surface.

Often called the ‘Goldilocks Zone’, the Habitable Zone is a hypothetical region around a star where the conditions are just right to be considered potentially habitable for life. In that region, a planet’s surface temperature could be in the range needed to harbor liquid water on its surface. The energy from the star, together with the greenhouse effect generated by the planet’s atmosphere, would produce a surface temperature between 0 and 100ºC (between 32 and 212ºF).

The Habitable Zone is the region around a star where the conditions could potentially be suitable to sustain life on a planet within this region, for example allowing the presence of liquid water on its surface. Credit: ESA/Hubble, M. Kornmesser

Hubble’s observations have shown us that planets are being formed around many more stars than previously thought, increasing the possibility that life could exist somewhere out there in the universe, or even in our own galaxy, the Milky Way. In the future, Hubble could possibly find hints of life by studying exoplanet atmospheres. The chemical makeup of a planet’s atmosphere leaves a unique fingerprint on the starlight that passes through it, which Hubble can investigate.

In 2017, an international team of astronomers used Hubble to look for atmospheres around planets orbiting within or near the habitable zone of the ultracool dwarf star TRAPPIST-1, 40 light-years away. Of the seven Earth-sized planets that orbit TRAPPIST-1, three of them orbit within the system’s habitable zone.

Habitable Zone. Credit: ESO/N. Bartmann/

In 2019, in an exciting discovery, Hubble data were used to detect water vapor in the atmosphere of a Super Earth within the habitable zone of its host star. K2-18b is eight times the mass of Earth and at its discovery was the only exoplanet known to have both water and temperatures that could support life.


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