When originally formed it was a ball of very hot liquid. Since then it has been gradually cooling and solidifying. The inner core gradually increases in size, and the outer core shrinks. Given enough time, it will eventually be solid all through.
But the planet won’t be around forever. The sun is predicted to become a red giant. Eventually it will expand beyond the orbit of Earth, and swallow up whatever is left of the planet.
Question is, will the Earth have time to solidify before this happens? Will the Sun cooling have any effect on the Earth’s gradual solidifying?
There is an alternative scenario where the planets recede from the Sun as it loses mass, so never get swallowed. But for this question lets assume that they remain in their current orbits.
Well the planets are receding, so it’s not an “alternative scenario”. However, it’s uncertain if the Earth will recede far enough to avoid being engulfed. Every time I see someone do a study on this, they seem to contradict the last study.
Anyway, what you’re really asking is Will the Earth solidify before the sun becomes a red giant? I believe the answer is no, it’s still be partially liquid.
Can the earth ever completely solidify? I don’t know the triple point of iron, but the center of the earth may be under enough pressure to keep it liquid no matter what the temperature.
As the OP noted, the inner core is currently a solid. It is already at a temperature of over 5,000K. Would an expanded sun increase that significantly? It doesn’t seem likely since the sun’s surface is only about that temperature.
The earth may stay hotter longer than you think. The heat inside the planet is not only due to “primordial heat” (the heat left over from the early formation days) but also due to on-going radioactive decay in the mantle.
A follow-up question: If Terra was flung to interstellar space on a path far from other suns, would it freeze solid, and if so, how long would that take?
As to the first question, yes, the Earth will freeze solid one day, assuming it is not engulfed during the red giant phase or suffers a collision powerful enough to destroy it. The Sun will eventually turn into a black dwarf and the radioactivity in the planet’s rocks will eventually run out, reducing the temperature of the Earth to something near that of the microwave background.
This article - the Timeline of the Far Future - describes some events that will affect the Earth’s temperature in the future.
For instance the Moon will probably collide with the Earth at around 150 billion years, and the Earth would hit the frozen Sun at 100 quintillion in the unlikely event it has not already been stripped away by external encounters.
If the sun would last forever, then the Earth’s core would solidify in roughly 91 billion years (according to an article from the National Geographic Society, I haven’t tried to do the math myself).
The sun will not last forever, though. It will become a red giant and will completely destroy the Earth in roughly 5 billion years.
So unless something comes along and flings the Earth outside of the Solar System, the Earth is doomed to die long before its core ever gets a chance to solidify.
The effect of the Sun’s heat is to keep the surface of the Earth at a temperature of around 300 K. The core of the Earth is at around 6000 K. So while the Sun does have an effect on the internal temperature of the Earth, it’s a very small one.
To clarify this, because it seemed wrong to me – the Moon’s orbit is slowly getting larger, and Earth’s day is getting longer, due to tidal effects. So the Moon is getting farther from the Earth. Without outside interference, the Moon would reach a (larger) stable orbit when the Earth becomes tidally locked to the Moon, and no collision would be possible. What that article is proposing is the outer atmosphere of the expanding red giant phase of the Sun would reach the Earth-Moon system and frictional drag caused by the Moon plowing through that atmosphere would slow it down and cause it to spiral into the Earth (before the Earth spirals into the Sun). As mentioned, there’s a lot of uncertainty about exactly how large the Sun will be in its red giant phase, so it’s not a sure thing that this will happen.
I don’t think that is what they are saying. The red giant phase is quite short, and only five billion years in the future. I think the collision mentioned on the Wikipedia page will be caused by gravitational radiation.
Ok, I looked up the citation given for the Wikipedia statement. Seems we’re both wrong (which means the Wikipedia statement should be clarified). This is what the original source says:
So it’s solar tides that he’s claiming will cause the Moon’s orbit to decrease in size.