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What Is Meant By “Habitable Zone” And How Do We Define It?

The term “habitable zone” is bandied about quite often, but what exactly does it mean? And do they apply to just stars, or are there larger HZs in the cosmos?

In the past decade, the number of planets beyond our Solar System – aka. extra-solar planets (or exoplanets) – has grown exponentially. In fact, as of today, a total of 3,925 exoplanets have been confirmed in 2,926 star systems, with another 3,389 candidates awaiting confirmation. SCIENCE of planets space science universe science

Much of the credit for these discoveries go to the Kepler Space Telescope, which was used to confirm the existence of 2338 exoplanets (with another 2423 awaiting confirmation) between when it began service on May 12th, 2009, and when it exhausted the last of its fuel on November 15th, 2018. SCIENCE of planets space science universe science

Within this large collection of extrasolar planets, just 49 have been designated as “potentially habitable” by astronomers. That’s just over 1% of the total data sample, which would seem to suggest that life-bearing planets are extremely rare in our Universe. SCIENCE of planets space science universe science

But when we take a step back and examine what is meant by “potentially habitable” and what goes into making that determination, we find that a lot of assumptions are involved. For one, we are searching for life based on “Earth-analogs” which could be severely limiting. SCIENCE of planets space science universe science

Second, there are many unknowns and ill-defined parameters when it comes to how “life as we know it” emerged, not to mention what kinds of conditions it can survive under. When we factor that into our search for life, we find that our estimates might be on the generous side. SCIENCE of planets space science universe science

The only way to clarify the whole issue is to examine what is meant by habitability, what goes into determining it, and keeping this in mind as we conduct future surveys for extrasolar planets. SCIENCE of planets space science universe science

Circumstellar Habitable Zone:

Also known as the “Goldilocks Zone”, a circumstellar habitable zone (habitable zone, or HZ for short) refers to the distance from star where a planet will experience temperatures between 273 K and 373 K (0 and 100 C; 32 and 212 F) – in other words, the temperature range where water is able to exist in liquid form. The range of this zone depends heavily upon the type of star. SCIENCE of planets space science universe science

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist’s impression of habitable zones around different types of stars. Credit: NASA/Kepler Mission/Dana Berry SCIENCE of planets space science universe science

For example, larger, higher-magnitude stars like O, B, A-type stars have wider habitable zones that are located at a relatively long distance. These stars are known as “blue giants,” which can be up to 1.4 million times as bright as our Sun and generally range from being about three to a few dozen (or even a few hundred) times the mass of our Sun. SCIENCE of planets space science universe science

These classes of stars are relatively rare, accounting for about 1 in 3,000,000 (O-type), 1 in 800 (B-type), and 1 in 160 (A-type) of the stars in our galaxy. F-type stars occupy a sort of middle ground, being blue-white in color and generally only a few times more luminous and massive than our Sun. These stars are more common, making up about 3% (1 in 80) stars in our galaxy. SCIENCE of planets space science universe science

Then you have stars more akin to our own, which fall into the G and K-type classes (yellow and orange dwarfs). These stars make up around 7.5% (1 in 13) and 12% (1 in 8) of the main-sequence stars in the solar neighborhood and have relatively tight and narrow habitable zones. SCIENCE of planets space science universe science

Last, but certainly not least, you have the class of low-mass, low-brightness, cooler stars known as M-type (red dwarf) stars. These stars range from being 7.5 to 60% the size and mass of our Sun and only get about 7% as bright. As a result, their habitable zones are rather narrow and very tight. They are also the most common type of star, accounting for about 85% of the stars in our galaxy. SCIENCE of planets space science universe science

What Is Meant By "Habitable Zone" And How Do We Define It?
The “Goldilocks” zone around a star is where a planet is neither too hot nor too cold to support liquid water. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley. SCIENCE of planets space science universe science

Despite these differences, the basic rule is generally the same across the board. If a planet is too close to its respective star, its surface water will evaporate rapidly and collect as vapor in the upper atmosphere, causing a “moist greenhouse” effect. SCIENCE of planets space science universe science

On the other hand, if a planet is too far away, the atmosphere will be cold and dry, and CO2 levels will remain high to the point that they would be toxic to Earth animals. This is well-illustrated by the planets of Venus and Mars, which orbit at the inner and outer edge of our Sun’s HZ. SCIENCE of planets space science universe science

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