Goldilocks Zone: Where Earth-Like Planets Might Exist

The Goldilocks zone is the region around a star where a planet could keep liquid water on its surface. Scientists call it the habitable zone. The nickname comes from the fairy tale, where the porridge is neither too hot nor too cold.

Why does water decide the zone?

• NASA defines the zone as the distance from a star at which liquid water could exist on a planet’s surface.
• Liquid water is essential for life as we know it.
• “Goldilocks zone” is an informal label. NASA and ESA both use it.
• Every star has a habitable zone. Its place depends on the star’s size and brightness.

What did UPSC ask?

UPSC asked about this in the 2015 Civil Services Preliminary paper. The term “Goldilocks Zone” is in the news in the context of the search for Earth-like planets in outer space. One wrong option was “the limits of the habitable zone above the surface of the Earth”. Others were shale gas and meteorites.

The idea is older than the nickname. Here is the short history.

What happened, step by step?

• 1853: William Whewell wrote about a “Temperate Zone of the Solar System”.
• 1959-60: Su-Shu Huang wrote articles in which the habitable-zone idea appears in the science literature.
• 1964: Dole called the zone the “ecosphere”.
• 1993: Kasting and colleagues built a climate model that put numbers on it. A 2014 review calls it the most widely used model.

What sets the two edges?

• The inner edge is where water is lost. A runaway or moist greenhouse takes over.
• The outer edge is where even the strongest carbon dioxide greenhouse cannot keep water liquid.
• No source names who first coined “Goldilocks zone”. One review says astronomers used the label in the 1970s.

The zone moves with the star. A brighter star pushes it outward. A dimmer star pulls it inward.

Where is the Sun’s zone?

• Kasting’s model gives about 0.95 to 1.67 AU. Kopparapu’s update gives 0.99 to 1.70 AU.
• One AU is the Earth-Sun distance. Earth sits at 1 AU, near the inner edge.
• Venus orbits at 0.72 AU. That is inside the inner edge.
• Mars orbits at 1.5 AU. That is inside the outer limit. Card 4 explains why Mars still lost its water.

What about other stars?

• Zones are wider for hotter stars.
• Red dwarfs are small, dim stars. They are about 73% of Milky Way stars.
• A red dwarf’s zone is very narrow and close. TRAPPIST-1 d circles at only 2% of Earth’s distance from the Sun, with a year of four Earth days.
• Sun-like stars are only 6% of stars. K dwarfs are 13%.

Distance only says the temperature could be right. A planet also needs the right air, protection and a calm star.

What went wrong on Venus and Mars?

• Venus: a thick atmosphere traps heat in a runaway greenhouse effect. Its surface is about 467 degrees Celsius. Models suggest Venus may once have had a shallow ocean.
• Mars: its atmosphere is thin, so liquid water cannot last long on the surface. Mars has no global magnetic field today.
• NASA’s MAVEN mission found that solar wind and radiation stripped the Martian atmosphere. Most of the gas ever present is lost to space.

What troubles planets around red dwarfs?

• Young red dwarfs start with powerful flares.
• Planets in the zone are exposed to extreme X-ray and ultraviolet light.
• Most such planets are tidally locked. One side always faces the star.
• NASA says being in the zone does not mean a planet is habitable.

These examples appear most often.

Which ones should you remember?

• Kepler-22b (2011): the first confirmed planet with a measured radius in the zone of another star. Its orbit takes 290 days.
• Kepler-186f (2014): an Earth-size planet about 500 light-years away in Cygnus. It gets one-third of Earth’s energy. NASA calls it an Earth-cousin, not an Earth-twin.
• Kepler-452b (2015): it circles a Sun-like star in 384.8 days. NASA calls it an Earth-cousin. It sits slightly outside the conservative zone.
• Proxima b (2016): it was found with the HARPS spectrograph on ESO’s 3.6 m telescope at La Silla, Chile. It is the closest known exoplanet, about 4 light-years away. Its orbit takes 11.2 days.
• TRAPPIST-1 e, f and g (2017): NASA says these three of seven planets lie in the zone. The star is 40 light-years away.
• TOI-700 d (2020): TESS’s first Earth-size planet in the zone.

What about K2-18b?

K2-18b is about 120 light-years away. Webb found methane and carbon dioxide in 2023. A 2025 claim of a possible life-linked gas was not confirmed by later analyses. This lesson does not call it habitable.

Different telescopes do different jobs. Together they find planets and test their air.

Which space missions matter?

• Kepler launched on 6 March 2009. It stared at about 150,000 stars in Cygnus and used the transit method. It retired on 30 October 2018 and left more than 2,600 planet discoveries.
• TESS launched on 18 April 2018.
• Webb launched on 25 December 2021. It is the only telescope able to study TRAPPIST-1 atmospheres at the needed precision.
• NASA’s tally of confirmed exoplanets was reported by NASA as 6,000 on 17 September 2025.

What has India done?

• The Physical Research Laboratory (PRL) runs the PARAS spectrograph at the Mount Abu 1.2 m telescope.
• ISRO says K2-236b, a sub-Saturn about 600 light-years away, was India’s first exoplanet. It was found in 2018.
• TOI-1789b (2021) and TOI-4603b (2023) followed. ISRO’s February 2025 release on TOI-6038A b calls it the fifth by PARAS-1 and PARAS-2 together.
• Two of these, TOI-1789b and K2-236b, are described as a hot Jupiter and a sub-Saturn.
• AstroSat joined a 31 May 2017 campaign on flares from Proxima Centauri.

Which newer missions will find more?

• ESA’s Plato will use 26 cameras to study planets up to the zone of Sun-like stars. ESA’s 2026 page says it is set to launch at the beginning of 2027.
• NASA lists the launch of its Roman Space Telescope as 30 August 2026. Its coronagraph can block starlight to image planets.

Possibly. Heat can come from inside a world, not only from its star.

How can a far-off moon stay warm?

• Europa, a moon of Jupiter, has a salty ocean under its ice. It holds about twice the water of Earth’s oceans.
• Europa lies far from the Sun, so its surface is frozen. Tidal flexing likely makes enough heat to keep the ocean liquid.
• Enceladus, a moon of Saturn, sprays water vapour through jets. Cassini discovered them in 2005.
• NASA’s Europa Clipper launched on 14 October 2024 to check for habitable conditions. It reaches Europa in 2030.

Is there a zone for whole galaxies?

• The Galactic Habitable Zone was proposed in 2001 by Gonzalez, Brownlee and Ward.
• It is the part of the Milky Way where an Earth-like planet can keep liquid water.
• A 2004 study put it in a ring 7 to 9 kiloparsecs from the galactic centre.

Good sources sometimes disagree. Here are the main cases.

Is Mars outside the zone?

Not by the usual numbers. Mars is at 1.5 AU and the outer limit is about 1.67 to 1.70 AU. The better lesson is that Mars lost its atmosphere and has no global magnetic field.

How wide is the Sun’s zone?

It depends on the model. Kasting gives 0.95 to 1.67 AU. Kopparapu gives 0.99 to 1.70 AU. Older estimates ranged from 0.725 to 1.24 AU. This lesson says roughly 1 to 1.7 AU.

What is new in 2025-2026?

• On 17 September 2025 NASA reported that its tally of confirmed exoplanets had reached 6,000.
• Webb results on TRAPPIST-1 e (September 2025) suggest it probably has no primary atmosphere. Webb found no Earth-like atmosphere on TRAPPIST-1 d (13 August 2025).
• K2-18b: a 2025 claim of a life-linked gas was not confirmed by later analyses.
• Plato is dated December 2026 on an older ESA page and the beginning of 2027 on a newer one. This lesson uses the newer.

What did UPSC ask in 2015?

UPSC CSP 2015 General Studies Paper I, Q45, asked what “Goldilocks Zone” is often seen in the news in the context of. The options were “the limits of the habitable zone above the surface of the Earth” and regions with shale gas. The others were the search for Earth-like planets in outer space and the search for meteorites. The official key for item 45 is (c), the search for Earth-like planets in outer space.