Amanda Maxwell

Apr 10th 2023

What Is a Habitable Zone, and Why Does It Matter?


An announcement by NASA in January 2023 introduced exoplanet discovery TOI 700 e to the world and brought the astronomical term “habitable zone” back to the daily news. But what is it, and why should we care? And what makes TOI 700 e an exciting find?

Searching the Galactic Habitable Zone

Knowing that there’s another potential home for us out there in the universe is a comforting thought. As a species, we’re also prone to wonder if we’re alone in the universe. Is there other intelligent life out there like us?

Searching habitable zones for Earth-like planets is a mix of curiosity and self-preservation. The first home-away-from-home planet outside our solar system was discovered in 1992, followed in 2017 by the discovery of seven planets in the TRAPPIST-1 system, which is named after the Transiting Planets and Planetesimals Small Telescope in Chile that first viewed them.

What is a Habitable Zone?

But what is a habitable zone exactly? The definition of “Earth-like” helps define what makes a zone beyond our solar system habitable. These exoplanets are defined as capable of supporting life, so therefore, they must possess liquid water on their surface and have oxygen in the atmosphere.

Water is definitely the holy grail in the hunt for exoplanets. Criteria for remote life detection also include carbon dioxide and, as described by a research article in the Proceedings of the National Academy of Sciences, biomarker gases (such as methane) that living organisms might produce.

With the definition “capable of supporting life” comes a strict set of conditions that must be met on the exoplanet surface. Including gases and liquid water, the planet must also be the right temperature, avoiding extremes that are too hot or too cold. This translates into the planet being neither too close nor too far from its star.

In other words, the orbit must be in a sweet spot, also known as a Goldilocks zone, for life to (theoretically) flourish. For example, The Planetary Society notes that only three of the seven TRAPPIST-1 planets lie in the system’s galactic habitable zone.

From Earth to Exoplanets

The most recently discovered exoplanet, TOI 700 e, is an Earth-like rocky planet orbiting star TOI 700. It was found a few years after TOI 700 d, which was spotted in 2020 in the same system by NASA’s Transiting Exoplanet Survey Satellite (TESS). TESS surveys stars looking for changes in their brightness that signal when a planet passes or transits in front of them. Since they take up more space and block more light, larger planets are easier to spot. From this data, astronomers can calculate a planet’s size and orbit around its star and glean more details of what the planet probably looks like.

The reason TOI 700 e is causing excitement is that, along with TOI 700 d, it makes TOI 700 one of only a few systems containing a cluster of habitable-zone planets. This might be due to the nature of the star at the center — it’s classified as an M dwarf star, which makes it cooler and smaller than the sun in our solar system. According to NASA, these red dwarfs tend to yield better results, since most of the exoplanets similar in size to Earth have been found orbiting them. Compared to our sun, M dwarfs are much more common, being cooler and less luminous, and so, the habitable zone tends to be closer to the star.

The type of star can also influence whether or not life might flourish on a nearby planet. Space notes that red dwarf stars can be very volatile and send potentially sterilizing bursts of energy at planets within their habitable zones. TOI 700 is described as a “quiet star,” meaning its Goldilocks zone might be just right for life to take hold and flourish on one of its planets.

Writing in The Conversation, the team that discovered the TOI 700 system notes that, since this star is relatively close to Earth, it gives a great opportunity to study how Earth-like planets evolve and support life. As exoplanet research evolves, this could help the search for life elsewhere.

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