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Taalnazi t1_j68fnr0 wrote

Or KOI-4878.01.

Likely to be in the habitable zone. No idea if it has got a large moon and a Jupiter in the outskirts, though. Star is also an F-type, meaning it stays stable for only 2-4 billion years, rather than our G-type Sun's 10 billion (though Earth will be in the habitable zone of it for only 5.5 billion years).

Kepler-90 similarly has an F-type star, but it has the same amount of planets.

There is "habitable", which you should understand as "habitable for life" (so even only for microbacteria-like life), and there is "Earth analog", which is what laymen are actually looking for.

• A stable star (G, K-type main sequence; or a M-type. Either way, the star's luminosity variability should be "quiet", ie. no more than 0.5%; our sun has 0.1%). The star should be older than 500 milion years. For alien life that's not just microbes, I personally think at least 3 billion, but not more than 6 billion years old, would be a safe bet.

• Eccentricity below 0.20 (for comparison, Earth has 0.0167, Mars 0.0934, and Mercury 0.2056). This is assuming a 24-hr orbit, 365 days of year, with a star like our sun. Higher, and water can remain liquid only temporary.

• A longer period, about 100 days at least (this is just my opinion). While I think shorter cycles could also be possible, I think it'd be hard for life to adapt to short and quick seasons. Perhaps it'd be adapted like a sleep cycle.

• Should be in the zone where the solvent is mainly liquid (ie; oceans; thus, habitable zone); or be a moon whose atmosphere is protected and whose surface is warmed, both by the host planet's magnetic field. Where this habitable zone starts and ends, depends on the star. For a G2V star like our sun, with water as solvent, it's about 0.8-1.15 AU (120-172 million km) away from the star.

• Have a relatively high density (which points to an iron core and thus likely a magnetic field).

• Equilibrium temperature combined with its atmospheric pressure needs to provide for a liquid solvent (ie. ocean) of water, methane, or ammonia.

• The planet should be below 10 Earth masses and between 0.8 and 2.5 Earth radii. Note that planets like these, if they have a radius leaning close to 2 or more, may be easily entered but hard to leave by rockets. Bigger and the atmosphere will be too dense. Smaller and it cannot hold onto its magnetic field for long, and thus also not its atmosphere.

• A large gas giant further away, to redirect meteors and comets away, also helps.

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