# Minimum distance a planet can remain habitable from a sun-like star?

This one source is stating a desert planet the size of Earth could maintain habitability at 0.38 AU from a sun-like star, but I highly doubt this is possible. What are some factors that could make this possible?

The source for my information is http://arxiv.org/abs/1304.3714

Also for a solar-like star can just use our sun as the standard for calculating luminosity and temperature.

• Out of curiosity, are you OK with tidally locked planets that might have a ring of habitability? Why are you skeptical about the inner limit that the paper you linked sets? Their methodology seems sound, and it passed peer review. It wouldn't be an Earthlike planet, but it would be habitable for living things - potentially people. Mar 12, 2016 at 4:59
• Yes i'm fine with tidally locked planets, a planet at 0.38 AU would very likely be tidally locked or atleast rotate very slowly like Mercury, infact that's actually about Mercury's average distance from the sun Mar 12, 2016 at 5:03
• Why do you doubt the paper's conclusion? Mar 12, 2016 at 7:10
• Habitable by humans or some other life form? Mar 12, 2016 at 11:23
• I doubt the conclusion because what would stop a planet orbiting at that distance from just becoming another Venus clone? Mar 12, 2016 at 17:01

I suggest reading this article which includes a mathematical formula for determining the habitable range around a main sequence star.

Basically, start of habitable zone (least distance) = $$\sqrt {L_{star}\over 1.1}$$ AU

AU = astronomical unit (150 million km)

$$L_{star}$$ is known as Absolute Luminosity of the star.

The formula for Absolute Luminosity is: $$L_{star}$$ = $$_{10} \left[{M_{bol} - 4.5}\over{-2.5} \right]$$

$$M_{bol}$$ is known as Bolomatric Magnitude of the star. In order to calculate it, you use this formula:

$$M_{bol}$$ = $$M_v + BC$$

Here $$M_v$$ is known as Absolute Magnitude of the star. $$BC$$ is Bolometric Correction constant. It depends on the stellar class of the star. There are 6 stellar classes, B A F G K M. Their bolometric correction constants are -2.0, -0.3, -0.15, -0.4, -0.8, -2.0 respectively.

In order to calculate $$M_v$$ (Absolute Magnitude), you use this formula:

$$M_v$$ = $$m_v - 5$$ x $$log{d\over10}$$

Here $$m_v$$ is the Apparent Visual Magnitude of the star (aka its visual spectrum). $$d$$ is the distance of Earth (our planet) from the star. This distance is measured in parsecs (one parsec is nearly 3.26 light years).

# In Short

You would need to know these things if you want to calculate the habitable around a star:

• its stellar class
• its apparent visual magnitude
• distance between Earth and that star

This site is about world building. While it is good and highly recommended to keep your world building compatible with real world values, scientific research and world building are two distinct things.

In order to set up a habitable zone around your star, I would suggest reading this wikipedia article. There is a table of stars and their habitable zone distances. You should see that and then make an estimate for the habitable zone around your star. Common sense and pattern recognition should land you fairly close to the real values for your star.

• Quick shortcut: Given that the star is Sun-like, all you have to do is plug in $L_\odot=L_{\text{star}}$. Mar 12, 2016 at 17:07
• @HDE226868: better yet, if the star is sun-like, just plug in a habitable zone nearly that of sun's. A little common sense and a bit of guesswork should do. It's world building after all, not an astronomy class. Mar 12, 2016 at 17:20
• Well, yes, but the Sun's habitable zone isn't well-known (surprising, I'd guess). But you're right (facepalm); that would be easier. Mar 12, 2016 at 17:22
• (facepalm) is always easier ;) (y) p.s. at least they know one thing about sun's habitable zone :D Mar 12, 2016 at 18:04
• Aren't there 7 stellar classes, with the O class having a higher luminosity and mass than the B class? Sep 15, 2017 at 2:55

While the answer "the nearest edge of the Habitable/Goldilocks zone" is a good one, Youstay Igo's advice about worldbuilding is perhaps the best part of an excellent answer.

A planet can be outside the habitable zone for millions of years before its atmosphere freezes or blows away. Tidally locked planets can have a mini-goldilocks zone of liquid water between the frozen and the gaseous, for some time. Cave-dwellers can hold onto atmosphere even after it's left the surface.

And so on. Sticking to the boring limits of generic science will give you a generic planet, rather than an interesting "on the edge" planet which would be fertile ground for the best stories.