Redefining the Goldilocks Zone for life in the Universe

As NASA’s Hubble telescope discovers the first evidence of water vapour in the atmosphere of an exoplanet, new research redefines the concept of the Goldilocks Zone to include not just distance from a host star, but a planet’s size too.

This illustration shows the lower bound for habitability in terms of planet mass. If an object is smaller than 2.7% the mass of Earth, its atmosphere will escape before it ever has the chance to develop surface liquid water (Harvard SEAS).

NASA has used data from the Hubble Telescope to make the first discovery of water vapour in the atmosphere of an exoplanet — a planet outside our own solar system. The planet K2–18b resides in the habitable zone around its parent star— a red dwarf roughly 110 light-years from Earth. This habitable zone around stars is an area often referred to as the ‘Goldilocks zone’ as it is believed to be ‘just right’ for life.

Just as the announcement of this major discovery was being made, a separate piece of research was being published which will also have significant ramifications for future assessments of an exoplanet’s capability to host life.

In the study published in the Astrophysical Journal, Havard researchers have redefined the size limit for planets to maintain liquid water for significant periods of time. The consequence of this readjustment means that smaller, lower-gravity planets may now in included in the Goldilocks Zone.

In fact, the study redefines the Habitable Zone as not just being a factor of orbital distance, but of planet size too.

This widens the net for researchers searching for life elsewhere in the Universe, in addition to expanding our knowledge of how atmospheres evolve on smaller planets.

The paper’s author, Constantin Arnscheidt, explains: “There are many other variables to habitability, including mass.

“Setting a lower bound for habitability in terms of planet size gives us an important constraint in our ongoing hunt for habitable exoplanets and exomoons.”

Water retention — the key to maintaining life

Maintaining water hasn’t likely been a problem for K2–18b, described as a ‘Super-Earth’ — an exoplanet with a mass somewhat in-between that of Earth and Neptune. But, for smaller planets and those closer to their host stars, hanging on to water, especially in liquid form and indeed any form of atmosphere is more of a challenge.

Generally, planets are considered ‘habitable’ if they are able to maintain surface, liquid water for a period of time that is long enough to allow the evolution of life. The hunt for such exoplanets is conducted with specific distances of certain types of stars. The cooler, smaller and less massive stars have habitable zones closer to them than larger, hotter, more massive stars.

This inner-edge of the Goldilocks Zone is generally defined as how close a planet can orbit its star before a runaway greenhouse effect is triggered, leading to the evaporation of surface water. Arnscheidt and his colleagues contest that this definition doesn’t hold for smaller, lower-gravity planets.

A runaway greenhouse effect happens when a planet’s atmosphere absorbs more heat than it loses via radiating into space. This prevents that planet from cooling — leading to uncontrollable heating which eventually leads to the oceans and surface water boiling off.

But as planets decrease in mass and size, this process is altered. As smaller planets warm, their atmospheres are able to expand outward more freely due to a lower gravitational pull. As these atmospheres become larger in relation to the radius our their planet, their capability to both absorb and radiate heat is also increased. Thus, meaning the planet can more easily maintain temperature stability and stave off runaway greenhouse effects.

This means that atmosphere expansion allows low-gravity planets to maintain liquid water, even when they orbit in close proximity to their host stars.

“But this planet is just right!”

Ganymede orbits the giant planet Jupiter. A saline ocean under the moon’s icy crust best explains shifting in the auroral belts measured by the Hubble telescope. Astronomers have long wondered whether Jupiter’s icy moons would be habitable if radiation from the sun increased. (NASA/ESA)

Once again in this cosmic tale, we can draw an analogy with the tale of Goldilocks and the Three Bears. The researchers have found that like baby bear’s porridge bowl — if a planet is too small their atmospheres are lost altogether and their surface water freezes or vaporises due to the lack of atmospheric pressure.

Again, a condition not conducive to life. Thus what the research suggests, is that there is a critical size below which planets could not possibly ever be habitable. This mean that a habitable — or Goldilocks — zone must be defined not just by orbit size, but also by planet size.

The researchers have set size critical size as being roughly 2.7% of the mass of Earth. Smaller than this, an atmosphere will escape well before surface water is even able to form. A process that is similar to what occurs on comets in the our Solar System.

The researchers were also able to estimate the radius of the habitable zones for planets within this critical limit. They considered scenarios modelled around both a G-Type star — such as our Sun — and a Red Dwarf — similar to the host star of K2–18b by pure coincidence.

Interestingly, the study may also allow astronomers to answer another long-standing question — namely could the icy moons of Jupiter — Europa, Ganymede and Callisto host life should they receive increased radiation from the Sun?

It would seem that from the conclusions laid out in Arnscheidt’s paper, said moons are too small in stature to maintain surface water even if they were closer to the Sun.

Robin Wordsworth, Associate Professor of Environmental Science and Engineering at SEAS and senior author of the study, concludes: “Low-mass water worlds are a fascinating possibility in the search for life, and this paper shows just how different their behaviour is likely to be compared to that of Earth-like planets.

“Once observations for this class of objects become possible, it’s going to be exciting to try to test these predictions directly.”