I am building a solar system with a k5v star at its center. Here are the some important stats for my star:

Mass: 0.70 M

Radius: 0.67 R

Luminosity: 0.15 L

Surface gravity (log g): 4.40cgs

Temperature: 4,526 K

Age: 6.1 Gyr

The first planet of my system is about as massive as Mars and has a similar composition of elements with Mercury. My planet's days are really long and due to a lack of a magnetic field and solar winds it lacjs an atmosphere. One thing i wanted out of this planet was to be close enough to its star that rocks and other minerals melt at the day side and solidify at the night side. This way, the one side of the planet is always a molten ocean of lava while the night side is vast cold valley of frozen magma. As some of you might guess this situation sounds similar to Mercury in real life (extremelly hot by day and really cold at night). However, Mercury's day side isn't covered in magma as the minerals in its surface have a higher melting point that the heat Mercury recieves from the Sun, so and even smaller type of star might appear to be too challenging. I though of solving this by moving the planet even closer to its parent star but there is a limit to how close a planet can be before the star's gravity tears the planet apart.

So to sum it up, how close should my planet be for it to be plausible (in terms of AU if possible)

  • $\begingroup$ We can't answer if you don't tell us the surface composition $\endgroup$
    – L.Dutch
    Aug 14, 2022 at 9:50
  • $\begingroup$ "...has a similar composition of elements with Mercury." Imagine a planet the size of Mars with the same elements you would find on the surface of Mercury: magnisium, silicon, iron etc. $\endgroup$ Aug 14, 2022 at 9:59
  • 2
    $\begingroup$ Don't get to caught up in the the whole "no magnetic field = no atmosphere" myth. Venus has no magnetic field $\endgroup$
    – James K
    Aug 14, 2022 at 10:48

1 Answer 1


Here is a planet temperature calculator http://home.ustc.edu.cn/~baishuxu/planettempcalc.html

Let's put in some numbers, with the aim of getting a surface temperature of about 2000K: I find that L=0.15, An albedo of 0.1 (comparable to the moon or Mercury) and a distance of 0.007AU gives temperatures in the right region. That is about 1 million km.

Now the Roche limit for such a star, and a dense, iron rich planet like Mercury is only about 500,000 km so this planet should be safe, for the time being.

A year on this planet is about 6 hours.

It is also certain to be tidally locked.

For a real life example of such a planet, look at Kepler 78b.


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