Raising a Solar System: A parenting guide (I)

Being the parent of a system of planets, asteroids and comets can feel overwhelming sometimes... but it also has its rewards. Here you have some tips for family harmony.

Artistic impression of a Solar System in the making. Credit: NASA

Nobody told you that being a star would be easy. To begin with, you have to take control of all the hydrogen gas in your core, careful not to burn it too fast or too slow if you want to win the eternal fight against gravity. But if you have a solar system, things get much more complicated. You may end up exhausted trying to keep the harmony among all your offspring.

Nevertheless, raising a family can also be very rewarding... You are the centre of your descendants' life after all! Sol, a star with numerous children, shares his secrets for a happy family life.

Tip #1: Maintain the order from the beginning

The creation of a Solar System is necessarily chaotic. It is quite unpredictable where dust is going to accumulate around you to form bigger and bigger clumps. Newborn planets tend to be pushy, clearing paths in the protoplanetary disk as they grow and bullying smaller bodies. Collisions are unavoidable, even between relatively large bodies –a few kilometers wide, or even more. "At first, we had a lot of conflict, with impacts happening all the time", Sol remembers. "Uranus, one of my sons ended up lying on its orbital plane and never recovered. And my daughter Venus is still spinning upside-down."

To prevent disputes as much as possible, it is very important to get things in control from the moment the first bodies are formed. Do not wait until the nebula is dispersed: As soon as planets appear, start arranging them, as well as their smaller siblings. "Some parents prefer to wait for things to settle for themselves", our expert says. "However, I don't like this approach, because if they get too messy, it is much harder for the children to accommodate to the new structure."

It is important to clear up and organise things from the beginning. In the image, an artist's impression of a young star with a protoplanetary disk gap created by a newborn planet. Credit: ESO

Of course, the distribution will be different if you are a single-parent or have any partners. "Since I have no companion, I simply keep all my children around me", Sol explains. "But if there are more stars in the family, you can explore other possibilities. My neighbours, the Alpha Centauris, are a triple system, and they have their planets under the care of one of the parents, Proxima. I have also met couples that prefer to have their lot surrounding the two of them. Nothing is carved in stone."

According to the expert, it is advisable to have this organisation more or less completed in the first few hundred thousand years. "It took me some time to figure out a system that worked, but thanks to that, we have lived quite peacefully for the last four billion years."

Tip #2: Keep rocky planets close, and giant planets at a distance

Sol keeps his rocky planets in smaller orbits than his giant planets. "The little ones need more attention and, moreover, for the big ones it may be harmful to stay too close", he explains. "To grow up properly, giant planets must remain beyond the frost line." He refers to the distance at which volatile ices stay solid. Without them, giant planets won't be able to get to the size necessary to retain the hydrogen and helium around their solid core.

However, once they have formed, you can let planets migrate and change orbit. "My biggest children were actually born further away, but they moved inward later, when they were fully formed", he says. "Of course, this implied a rearrangement, but nothing so dramatic as it would have been if they had grown up closer to me." In any case, he warns that giant planets should still maintain some distance, for the good of both the child and the parent. He is thinking of cases like the one of his friend HD 189733, whose planet is so near that it is inflated due to the effect of gravity and risks being completely evaporated. "HD 189733 herself is affected by strong flaring, and she suspects it also has to do with the interaction with her son, although this has not been proved."

Sol's family organisation: rocky planets in inner orbits, giant planets in outer orbits, asteroids and dwarf planets in belts and comets in clouds. Credit: Encyclopedia Britannica

Tip #3: Group the minor bodies

Minor bodies (dwarf planets, asteroids and comets) will get the worst in case of conflict, so it is wise to keep them out of the way of the big ones. Sol has grouped the majority of asteroids (along with the dwarf planets) in two belts. "This helps me keeping them under control", he says. "And the biggest brothers can also give me a hand."

This does not mean that all minor bodies will be happy with the arrangement. "There are always rebels, you know", Sol sighs. "I myself have a group of asteroids that I must keep an eye on at any time. They are real troublemakers." As an example, he mentions the time one of them hit Earth and caused a massive life extinction. "They call themselves NEOs. They say that stands for "near-Earth objects", but I know they chose it because of that Matrix movie."

Comets are even more difficult to keep in check. "They are small and vulnerable, but also sociable and curious", our star explains. "I have managed to hold the majority of them in two clouds, the largest one surrounding the whole system. Still, from time to time, one of them has to come to me to tell me how things are going." A visit full of risks, as it turns out. "I cannot forget how my little Shoemaker-Levy 9 fell into my biggest son, Jupiter." Sol's voice is muffled with emotion as he remembers the tragical event. And this was not the only traumatic incident: Sol admits to have lost many comets that have come to close to him in the course of the years. "I have tried to explain to them the gravity of things, but they simply don't listen."

Impact of comet Shoemaker-Levy 9 (left) with Jupiter, observed in infrared light. The body on the top right is Jupiter's moon Io. Credit: Max Planck Institut für Astronomie

Once organisation has been achieved, it is time to manage the little conflicts that may arise. We will talk about it in our next post.