This is about planets on which life as we know it could arise, and how different they might seem to Earth. By "life as we know it", I mean life sharing the same chemical basis as our own - requiring liquid water and employing the same amino acid / nucleic acid chemistry, etc..

Obviously, there is the matter of a "Goldilocks zone" within which water can exist in liquid form on a planet's surface. But that's not a specific distance (or distance range) from the host star... it depends on how brightly the star is shining. So, some "Earth-like" planets might have a very different year length than Earth.

Then there is the matter of a planet's gravity... enough to hold an atmosphere - also to allow liquid water to exist on its surface.

There is also the composition of the planet and its effect on density and therefore gravity, not to mention availability of the required elements for life.

Presumably some variables could be traded off against others - a hotter star might have its Goldilocks zone further away than a cooler one. A planet with higher gravity could contain an atmosphere and host liquid water at a higher temperature than one with lower gravity.

The point of my question is: how much can the various parameters like type of host star, distance from host star, composition of the planet itself, size/density, rotational period (length of day), etc. vary and be traded off one for another and still leave an environment suitable for the origin and evolution of life as we know it?

For example: can a star emit too much radiation at the wrong place in the spectrum e.g. too much UV or perhaps X-rays at a distance close enough to keep adequately warm? How long or short could a year be on a planet that can host life? How strong or weak might its surface gravity be? What could it be made of besides iron, nickel, and silicate rocks?

Disregarding the local flora/fauna, how unfamiliar could the surface conditions of such a place seem to us?

  • $\begingroup$ This comment links to this paper which is worth a read! $\endgroup$
    – uhoh
    Jan 17, 2018 at 12:29

1 Answer 1


The term lacks, as far as I've read, an IAU definition, so its use has been vague.

In general, it's being applied to extrasolar rocky bodies under about 4 Earth masses, or with diameters under about 1.5 earth diameters, that are in or near the habitable zone.

NASA has referred to a number of bodies as "Earth-Like"... the most recent is Kepler 186F. It's estimated to be 1.11 earth diameters, and at the outer edge of the habitable zone.

But with Kepler 452B, the more cautious term "cousin to Earth" is used... despite being 1.6 Earth diameters across, in the habitable zone, and having a 385 day year, and orbits a sun-like star.

NASA, on their Planetquest site, defines it thusly:

About a dozen habitable zone planets in the Earth-size ballpark have been discovered so far -- that is, 10 to 15 planets between one-half and twice the diameter of Earth, depending on how the habitable zone is defined and allowing for uncertainties about some of the planetary sizes.

It also mentions (in a long-winded way) that to be Earth-like requires it to be in the "Goldilocks Zone" (a.k.a. the habitable zone).

So, in general, a firm definition is lacking, but the weak one is 0.5 to 1.5 Earth diameters, presumed to be a rocky body, and in the habitable zone of the parent star.


  • $\begingroup$ Interesting. What I was looking for was information about how different a star might be to Sol (bigger, smaller, hotter, cooler, older, younger) but still offer a "habitable zone", how much such differences might affect length of year, apparent size/color of host star in the daytime sky, etc., how much the appearance of a daylight or nighttime sky might differ due to differences in emitted light from the parent star combined with differences in a still-habitable planetary atmosphere, etc.. $\endgroup$
    – Anthony X
    Aug 23, 2015 at 21:45
  • $\begingroup$ Every size class V and most size class IV stars above R should have a reasonable hab zone... Size IV and size V types R & L should be able to have a sufficiently warm body that's tidelocked. (A few have detected bodies in such orbits. Sun-like stars are types F/G/K size V. I don't know if size I, II, and III are being looked at. Size I and II are expected to be too short lived for life, and size III is considered marginal from what I've read. Temperature and hab zone is an inverse square proportion, and it's possible that III's could have planets. $\endgroup$
    – aramis
    Aug 24, 2015 at 10:49

Not the answer you're looking for? Browse other questions tagged or ask your own question.