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I'm designing an Earth-like planet that has a mass of 1.9 that of Earths' and a gravity of 1.44 g. It is orbiting around a k7v orange dwarf star at a distance of 0.47 AU and with an orbital period of 0.39 Earth years.

Now, due to a number of collisions that my planet went through during it's formation, it is permanently tidally locked, but can still support life thanks to strong wind currents that carry heat around the globe.

However, I've realized that in order for my planet to have an atmosphere capable of carrying heat effectively, it also needs to have a strong magnetic field to protect the atmosphere from the star's solar flares, something that could be difficult considering how slowly the planet needs to spin around itself in order to be tidally locked.

If we assume that the planets iron core makes up 49% of the planet's mass, will my planet be capable of maintaining a magnetosphere long enough to sustain multicellular life?

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    $\begingroup$ Venus has no magnetic field yet it has an atmosphere $\endgroup$
    – L.Dutch
    Oct 12, 2022 at 19:39
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    $\begingroup$ @L.Dutch, Although technically true, the lack of magnetic field has allowed the solar wind to strip the planet's atmosphere of its lighter elements. The described mechanism is that strong bombardment of ions from the Sun caused water to dissociate into oxygen and hydrogen. The light-weight hydrogen floated to the outer atmosphere, where the solar wind blew it out to the heliopause, while the oxygen bound with carbon to make the current CO2-rich atmosphere. $\endgroup$ Oct 12, 2022 at 21:20
  • $\begingroup$ Aside from flares, this article also mentions that the larger amount of ultraviolet radiation from orange dwarfs would tend to ionize more molecules in the atmosphere, so a strong magnetic field would be needed to prevent the ions from being carried away by the star's solar winds. $\endgroup$
    – Hypnosifl
    Oct 13, 2022 at 3:11

2 Answers 2

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The Good, the bad, and the ugly...

  1. The Good

You're dealing with a dwarf star. Yes, solar flares are a problem, but the star isn't as capable of flares (or a solar wind) like our own. (I think... my betters will please correct me if I'm wrong about this.) This means you don't need a magnetic field as strong as Earth's

  1. The Bad

Your planet rotates once each year. That's a requirement of being tidally locked. Because your planet rotates it can believably set up enough rotation in a liquid core to cause a magnetic field, but scientifically it won't be much of one.

Robert's answer makes a point I didn't know about, but I do have a difficult time believing the core can spin that much faster than the planet rotates. In other words, the two ideas together will rationalize a somewhat stronger magnetic field... but you're still not going to be at Earth's magnetic field unless you give up the tidal lock. Again, my betters may say I'm wrong, but radioactive convection + friction against the underside of the mantle compared to the gravitational force maintaining the lock... Hmm....

  1. The Ugly

While it's true that we have one and only one data point to work with when it comes to habitable planets in the universe (Earth) which usually means you have a lot of latitude to work with (because who can say you're wrong?) this one feels like you might be coloring too far outside the box. You have a weak star, a tidally locked planet, and you want it to be Earth-like.

Generally speaking, if you write a good story people will forgive coloring outside the box while if you write a bad story all the scientifically-accurate facts in the world won't save your story. This combination feels like you'll need a good story — but that's a comfortably long way away from "it can't be done."

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    $\begingroup$ I could see the combination of radioactive convection and the lack of solar flares making this a workable solution. Personally, I'm more dubious about a survivable ecology existing on a tidally locked planet. The one example we have of that says that unidirectional convection + orbital velocity would create a permanent planetary hurricane, where the atmosphere blows around the equator once every four days. But, hey, whatever makes a good story. $\endgroup$ Oct 14, 2022 at 16:25
  • $\begingroup$ Solar flares are much, much worse for red dwarfs than a star like our own. Not only are they more frequent, but more violent, owing to the fact that red dwarfs are completely convective and when you get more plasma moving more, you get more magnetic field, which leads to more line breaking from differential rotation, stronger flares, more frequent CMEs, etc. Not to mention sunspots block up to 40% of emitted light at times making the habitable zone shift with time. Wikipedia has a nice article about all this $\endgroup$
    – Justin T
    Oct 26, 2022 at 22:38
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Absolutely. There is still quite a bit of mystery about why we've maintained our magnetic field, but the leading theories are that radioactive elements in the core are creating magma currents that keep our core spinning. This process doesn't require the planet itself to rotate, just the core to rotate within the planet.

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  • $\begingroup$ Even if there is indeed quite a mistery around it I would like to specify that one of the main answer would be our moon. Without our moon it is almost accepted that our core wouldn't have such activity today whether it is that our planet would not rotate as much or because the tidal forces would not act on the core $\endgroup$
    – ohpif
    Oct 14, 2022 at 9:57
  • $\begingroup$ Granted, that is another theory, but it's not currently the most popular one. When asking "is it possible," less popular theories only have value if you're trying to come up with an alternate path. This question is asking if the popular theory still applies if the planet is tidally locked, and it does. If the OP wishes to go the route of the planet NOT having a magnetic field, that could also be rationalized. $\endgroup$ Oct 14, 2022 at 16:13
  • $\begingroup$ The leading theory for the Earth's magnetic field is dynamo theory, which is crucially dependent on the Coriolis effect on convecting magma, and the Coriolis effect is completely dependent on 1) the speed of the convection and 2) the rotation of the body. So it very much seems that this whole process is diminished significantly if the body is not rotating as much. $\endgroup$
    – Justin T
    Oct 26, 2022 at 22:51
  • $\begingroup$ @JustinT, I'm not sure if this supports or detracts from my argument. The magnetic field is generated as a differential between the core and the crust's rotational speed, so I would think that the rotation of the crust cancels out. $\endgroup$ Oct 26, 2022 at 23:57
  • $\begingroup$ @RobertRapplean The rotation of the crust does not cancel out and the differential between the core and the crust is not what creates the magnetic field; rather it is fluid that would otherwise be moving in straight lines because of convection that become spirals because of the Coriolis effect, which is dependent on the rotation of the earth; no rotation would mean no Coriolis effect, meaning no dynamo effect. Granted there is rotation here, but the extent that you rotate directly correlates to magnetic field. This hopefully helps illustrate that $\endgroup$
    – Justin T
    Oct 27, 2022 at 0:54

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