I was wondering whether there were any objects that orbit the "goldilocks zone" besides our home planet earth, and how the goldilocks zone of our solar system is defined.
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2$\begingroup$ Our Moon is also in the Goldilocks zone. $\endgroup$– PM 2RingCommented Sep 4, 2019 at 9:15
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$\begingroup$ Probably good to note, habitable zone isn't synonymous with "livable planet", if there's no magnetosphere or atmosphere, the Goldilocks zone is still very volatile for life. As PM points out the Moon is a good case study for that. $\endgroup$– Magic Octopus UrnCommented Sep 5, 2019 at 16:20
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2 Answers
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From the Wikipedia article Circumstellar habitable zone, which is just another name for the Goldilocks zone:
Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units, though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However their atmospheric conditions vary substantially. The aphelion of Venus, for example, touches the inner edge of the zone and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to 462 °C (864 °F) at which water can only exist as vapour. The entire orbits of the Moon, Mars, and numerous asteroids also lie within various estimates of the habitable zone.
See also these questions:
- What is the Habitable zone all about?
- Why the Circumstellar Habitable Zone is defined as it is, if life could be possible outside of it?
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$\begingroup$ as expected from our man @SpaceBread approve $\endgroup$ Commented Sep 5, 2019 at 7:21
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There are many definitions for the "Goldilocks zone" or Habitable zone. If you want liquid water on the surface then that can happen for a wide variety of atmospheric pressures (totally unknown for the vast majority of exoplanets) and temperatures. In fact Mars is considered to be in the habitable zone using only this criteria.
More strict criteria exist for a planet to have liquid water on the surface making some assumptions about their atmospheres and other needs.
For the conservative estimate we have 1 planet of the Solar System (Earth), 16 confirmed exoplanets (Proxima Centauri b, Gliese 667 Cc, Kepler-442b, Kepler-452b, Wolf 1061c, Kepler-1229b, Kapteyn b, Kepler-62f, Kepler-186f, Luyten b, TRAPPIST-1d, TRAPPIST-1e, TRAPPIST-1f, TRAPPIST-1g, LHS 1140 b and Kepler-1638b) and 1 unconfirmed exoplanetary candidate (Teegarden c) residing in the habitable zone of their stars.
But this doesn't mean that those 16 confirmed exoplanets are habitable at all. There are many more things to be considered (all with extreme uncertainties by now). One thing that is very easy to pin-down is the fact that the planet might be tidally locked or not to its star, meaning that one face of the planet might always be in broad day light and the other in perpetual darkness. We know that climatic conditions could be so extreme as to make the planet sterile just because of that (assuming what we know about life in the cosmic contex which is very limited). Of those 16 confirmed exoplanets in the habitable zone 14 orbit an M-type or K-type star, whose habitable zones are so close to the star itself that tidal locking is a highly probable state for the planets.
The remaining 2 non tidally-locked habitable-zone confirmed exoplanets are Kepler-452b and Kepler-1638b. Of them Kepler-452b is the current most promising candidate for life of the 4109 confirmed exoplanets by now.
But here I have to advise you again. We know very little about the conditions that make life possible (since we are still figuring out how it was originated in our planet), we don't know also if there are many alternative paths to life or just ours. We don't know to what extent life can adapt to extreme enviroments and extreme changes, we don't know the history of these planets (maybe the star erradicated life in the past, maybe the planet migrated to the habiltable zone just recently, maybe they didn't have water on the surface even if in the case they did they would have it in liquid state, maybe they didn't have a magnetosphere or an ozone layer to protect them from all sorts of radiation that could disrupt any complex molecule of any conceivable life-form, maybe the planet has too much gravity for life to evolve...). There are many many unknowns and we are just starting to explore in this field of science, with such primitive criteria and with so little data we aren't going to be able to say if these planets are trully habitable (and even less if they are inhabited) for more than a century (in my opinion), but who knows...
