According to this work and worldbuilding pasta’s predictions on the water condensation inner limit for stellar habitable zones, a star like the sun cannot have a planet with liquid water on its surface any closer than 0.83 AU. This is because the star would initially be too luminous and hot (more than 95% the sun’s current stellar flux) to allow water to condense on the planet’s surface.

Various inner edge boundaries of the Habitable Zone (HZ) for different types of stars
enter image description here
Image from Water Condensation Zones around Main Sequence Stars, by Turbet, et al., p. 14, fig. 8, arxiv.org/pdf/2308.15110: Various inner edge boundaries of the Habitable Zone (HZ) for different types of stars

Let’s say I wanted to have an asynchronous slow-rotating planet at 0.73 AU from a sun-like star, and I wanted water to be able to condense on its surface after the initial hothouse state so that the planet would eventually have oceans and be warm but habitable. Could I have its parent star initially form to be less luminous than the sun initially did, but 4.6 billion years later be just as luminous as the modern sun?

BONUS: Would this method also work for other stars (i.e. K-type, other G-types and F-type stars)?

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    $\begingroup$ Have you considered the possibility of planetary migration? Planets don't necessarily stay in the same orbits over the course of billions of years. $\endgroup$
    – Monty Wild
    Commented Jun 23 at 23:42
  • $\begingroup$ You don't mean "less bright," you mean "lower energy output." Brightness is (literally) in the eye of the beholder. My gut tells me the answer is no because fuel is consumed and you can't build up to a point of greater consumption (violates the laws of thermodynamics), but I'm no expert in nuclear physics where fusion might lead to something that can temporarily increase energy output. Nonetheless, this isn't a worldbuilding question. You should ask this on Physics or, probably better, on Astronomy. $\endgroup$
    – JBH
    Commented Jun 23 at 23:53
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    $\begingroup$ Stars start lesser bright and then they become slighter and lighter until a red giant phase. $\endgroup$
    – Gray Sheep
    Commented Jun 24 at 3:29
  • $\begingroup$ @MontyWild I have, but I am worried it may complicate my system too much, especially since something that would cause inward migration may cause the planet to have higher eccentricity and/or tidal stress. I would be worried this could cause the early oceans on the planet to boil away. $\endgroup$ Commented Jun 24 at 11:15
  • $\begingroup$ Our own Sun is "a star that initially started with less energy output than the sun" and then "evolved to be as luminous as the sun". And quite obviously it did it "in the same time period". $\endgroup$
    – AlexP
    Commented Jun 24 at 12:07

4 Answers 4


Yes... relatively speaking.

If we consider stellar evolution, stars begin their lives relatively dim, then as they progress through the main sequence, they become brighter, before they become a giant star.

The time spent on the main sequence is dependent upon the spectral class of the star. If we look at this table, summarised below, we can see that lifespan on the main sequence is dependent upon spectral class:

Spectral Class Effective Temp (K) Colour M/MSun R/RSun L/LSun Main Sequence Lifespan
O 28,000 - 50,000 Blue 20 - 60 9 - 15 90,000 - 800,000 1 - 10 Myr
B 10,000 - 28,000 Blue-white 3 - 18 3.0 - 8.4 95 - 52,000 11 - 400 Myr
A 7,500 - 10,000 White 2.0 - 3.0 1.7 - 2.7 8 -55 400 Myr - 3 Gyr
F 6,000 - 7,500 White-yellow 1.1 - 1.6 1.2 - 1.6 2.0 - 6.5 3 - 7 Gyr
G 4,900 - 6,000 Yellow 0.85 - 1.1 0.85 - 1.1 0.66 - 1.5 7 - 15 Gyr
K 3,500 - 4,900 Orange 0.65 - 0.85 0.65 - 0.85 0.10 - 0.42 17 Gyr
M 2,000 - 3,500 Red 0.08 - 0.05 0.17 - 0.63 0.001 - 0.08 56 Gyr

So, by having a star that is hotter than our sun (which is a G2 star), it will pass through the main sequence more quickly. It could quite easily begin its life relatively dim (at the orbit of a habitable world) compared to our sun at Earth's orbit, but end up with the same apparent brightness after a period of time.

  • $\begingroup$ Wouldn’t a typical hotter star still start off its life brighter than the sun did? $\endgroup$ Commented Jun 24 at 10:02
  • $\begingroup$ @casualworldbuilder Probably it would, but not necessarily. If it was smaller and hotter, it might start dimmer. $\endgroup$
    – Monty Wild
    Commented Jun 24 at 11:47

A Moving Target:

An alternative to reach the same goal:

What you are really worried about is the planet, and condensation there. The star is just the causative agent. So why can't you shift the planet? Your planet starts out in one orbit, then some event like a close call with [another star, planet, small massive object] causes your world to later shift to a different orbit. The lucky little planet gets a cool start, doesn't get roasted, and finally nestles into a warm, comfy orbit to incubate life.

  • $\begingroup$ I have considered this as well, although I would be concerned that to get a planet to change its orbit and move inwards, it would require something that may cause the planet to increase dramatically in eccentricity and/or tidal forces which may cause the early oceans to boil away $\endgroup$ Commented Jun 24 at 14:08
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    $\begingroup$ @casualworldbuilder There are always a lot of near-perfect outcomes needed for life like our world. Any orbits like you described would simply result in another uninhabited rock like thousands of others. There are fancy gravitational things that may gradually circularize an orbit (fairly hypothetical) or just the right collision (because anything perturbing your planet might perturb other things that smash into it). $\endgroup$
    – DWKraus
    Commented Jun 24 at 18:13

Sun probably itself did exactly this.


It is still not clear how we (our early forms) did back then, but it is more or less assumed that stars like ours generally do slowly increase their luminosity over their main sequence lifespan.


Just throw in some space dust. In the past the system was going throw a segment of space with remnants of earlier star system. It blocked part of the energy. Over time conditions changed.

I doubt people would care much either way though.


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