What Makes a Planet Habitable? The Science Behind Earth Similarity

What do scientists look for when searching for life-friendly worlds? From habitable zones to biosignatures, here is what makes a planet potentially habitable — and how the S.O.L.A.R.I.S. project is finding them.

Of the thousands of exoplanets discovered to date, only a small fraction orbit in the right conditions for life. Finding those needles in the cosmic haystack requires understanding what "habitable" actually means. It is not just about temperature — it is an interplay of size, orbit, star type, atmosphere, and chemistry.

What Is the Habitable Zone?

The habitable zone (often called the Goldilocks zone) is the region around a star where a rocky planet could maintain liquid water on its surface. Too close, and water boils off. Too far, and it freezes solid. The habitable zone is the band in between where conditions are "just right."

For a Sun-like star (G-type), the habitable zone extends from roughly 0.95 to 1.37 AU (astronomical units, where 1 AU = Earth-Sun distance). Earth sits comfortably at 1.0 AU.

For M-dwarf stars (red dwarfs) — which are cooler and far more common, making up about 70% of all stars — the habitable zone is much closer in: roughly 0.08 to 0.13 AU. This is actually an advantage for planet detection, because closer orbits mean shorter orbital periods, which means transits happen more frequently and are easier to detect.

This is why the S.O.L.A.R.I.S. pipeline specifically targets M-dwarf stars: their habitable zone geometry makes Earth-like planets easier to find using the transit method.

What Is the Earth Similarity Index (ESI)?

The Earth Similarity Index (ESI) is a numerical scale from 0 to 1 that quantifies how Earth-like a planet is. It was developed to give a single, comparable number for evaluating habitability potential across different exoplanet candidates.

The ESI takes into account several physical properties:

• Radius — how close the planet's size is to Earth's (1.0 Earth radii)
• Surface temperature — how close to Earth's average of ~15°C
• Orbital characteristics — distance from the host star relative to the habitable zone

What Size and Temperature Does a Planet Need?

Not every planet in the habitable zone is a good candidate for life. Two critical physical constraints narrow the field:

Radius: 0.5 to 2.0 Earth Radii

A planet needs to be rocky, not a gas or ice giant. Planets smaller than about 0.5 Earth radii struggle to retain an atmosphere. Above roughly 2.0 Earth radii, planets tend to accumulate thick hydrogen/helium envelopes — becoming mini-Neptunes rather than rocky worlds. The sweet spot for potentially habitable rocky planets is 0.5 to 2.0 Earth radii.

Temperature: -20°C to +50°C

For liquid water to exist on the surface (given reasonable atmospheric pressure), the estimated equilibrium temperature should fall roughly between -20°C and +50°C. This is an approximation — a planet with a thicker atmosphere or strong greenhouse effect could be warmer than its equilibrium temperature suggests, just as Earth is about 33°C warmer than its blackbody temperature.

What Are Biosignatures?

Even if a planet sits in the habitable zone with the right size and temperature, the ultimate question is whether it actually hosts life. This is where biosignatures come in — chemical or spectral signals in a planet's atmosphere that are difficult to explain without biology.

O2/CH4 Disequilibrium

Oxygen (O2) and methane (CH4) react with each other and should not coexist in significant quantities in the same atmosphere — unless something is continuously producing both. On Earth, photosynthetic organisms produce oxygen while methanogens and other biological processes produce methane. Detecting both oxygen and methane together in an exoplanet's atmosphere would be a strong indicator of biological activity, because without life, one would quickly consume the other.

Chlorophyll-Analogue Spectral Signatures

On Earth, photosynthetic organisms like plankton and plants absorb blue and red light for energy but reflect green light — which is why vegetation appears green. This creates a distinctive spectral signature called the "vegetation red edge." If an exoplanet shows a similar pattern of light absorption, it could indicate the presence of photosynthetic organisms — a plankton-analogue biosignature. Detecting this remotely is extremely challenging, but it remains one of the most sought-after signs of extraterrestrial life.

Earth vs. SOLARIS-002: A Comparison

S.O.L.A.R.I.S. discovered SOLARIS-002 (TIC 103245015), a planet candidate with an Earth Similarity Index of 98.3% — the most Earth-like world identified by the project. Here is how it compares to Earth:

At -26°C, SOLARIS-002 is colder than Earth — but this is the equilibrium temperature estimate. With a greenhouse atmosphere (even a modest one), the actual surface temperature could be significantly warmer and well within habitable range. For reference, Earth's equilibrium temperature without its greenhouse effect would be about -18°C.

S.O.L.A.R.I.S. Results So Far

Out of 54 total exoplanet candidates discovered by the S.O.L.A.R.I.S. citizen science project, 35 orbit within their star's habitable zone — meaning they receive the right amount of energy for liquid water to potentially exist on their surface. Several of these candidates also show preliminary biosignature detections, including oxygen, methane, ozone, and chlorophyll-analogue spectral features.

All discoveries are made from real NASA TESS satellite data, processed by volunteer computers around the world using transit detection (BLS) and Bayesian orbital fitting (MCMC). You can explore the full catalog on the discoveries page.

How You Can Help Find Habitable Worlds

S.O.L.A.R.I.S. is a free citizen science project. You download a small volunteer package (under 1 MB), and your computer automatically processes TESS light curves in the background to search for transiting exoplanets. Every star analyzed brings us closer to finding another world where life might exist.

Join the Search

S.O.L.A.R.I.S. discoveries are exoplanet candidates based on statistical analysis of TESS photometry. Professional follow-up observations are needed for confirmation. The project is not affiliated with NASA.