Two Earth-like planets collide at a (relatively slow) speed of about 1,000 kph. For all questions regarding the size and composition of these planets, just think about Earth - and these two planets are moving towards each other at the same speed.

1) First, they would begin to rip each other apart due to roche limits.

2) Then the two planets (what remained of them, that hadnt been ripped apart) would connect with each other.

3) Eventually, they would merge together and form one larger planet.

My question is, roughly what kind of time span would there be between stages 2 and 3? Stage 2 being the planets connecting with each other, and Stage 3 being one single spherical planet. So I want to know roughly how quickly gravity would act upon these two planets whilst it is forming them into one (mostly) spherical planet.

Are we talking hours/days or are we talking years/centuries?

We're talking hours to days.

A good deal of work has been done on protoplanet-protoplanet collisions, mainly focused on testing the Giant Impact Hypothesis for the formation of the Moon. A number of fluid simulations (many smoothed-particle hydrodynamics) have been performed, for varying angles of attack and initial relative velocities (see e.g. Canup 2012, Eiland et al. 2013).

The takeaway from those simulations is that the planets initially coalesce within half a day to a day. However, the resulting body isn't round; it's somewhat elliptical, even a bit pointy at the ends. Some models have tails of matter (typically one or two) attached at the ends, which, though tenuous, may form another body, i.e. the Moon. By the end of about 24 hours, there is a clear central body surrounded by this excess material, but it may take up to a month for it to regain its spherical shape - a key characteristic of a planet.

Other things to consider:

• It may take time for the interior of the planet to become differentiated, i.e. for it to take on a traditional planet-like structure. Even after coalescence, the cores may still be separated.


• Glancing, indirect collisions tend to produce more ellipsoidal shapes than direct collisions, even if there's a merger.


• There will still be debris orbiting for quite some time after the merger - again, perhaps weeks or months.


• The final body will remain quite hot for some time, with surface temperatures of perhaps up to 6000 K in the day or so immediately following the collision.

Please check out this link in case you find it interesting. It's about how the moon formed from a similar impact.

In the link above, it is assumed that there was an explosive collision (moderate at celestial standards) between Earth and Theia at an oblique angle. Despite such a collision, it is thought that it took 0.1 billion years to form the moon. A corresponding collision between your two planets would likely take longer, as the creation of the Earth itself (normally) took hundreds of millions of years.

A point about Roche limits: Roche limit takes effect 2.5 radii away from the larger planet. If these planets are equal in mass, they would merge into a central mass between them. This would basically be the same as forming a brand new planet from scratch.

Edit: I forgot to give you an actual answer - sorry lol. With little-to-no actual science to back this up (we don't know much about the formation of planets) I'm going to say between 0.5-1 billion Earth years. That's assuming that these two planets don't just turn into an asteroid belt or something, and that nothing else gets in the way. I'm also not accounting for bombardment of debris from the collision of these two planets, or the possibility of smaller moons forming.

I'd also like to point out that the probability of 2 celestial objects 'only' crashing into each other at 1000 km/h wouldn't be much of a collision. Is this being done deliberately? If not, incredible luck.

Helpful links:

https://en.wikipedia.org/wiki/History_of_Earth#Solar_System_formation
http://abyss.uoregon.edu/~js/glossary/roche_limit.html