How To Make a Planet Into a Star
October 5, 2009
I miss Arthur C. Clarke. He died more than 18 months ago at age 90, but the great science-fiction writer left more than 30 novels and dozens of short stories that described a mostly optimistic vision of mankind’s exploration of space and his responsible use of technology. His most famous work, 2001: A Space Odyssey, was made into what many consider the best science fiction movie of all time.As a writer, Clarke was no Hemingway. But the strength of his writing came not from his elegant style or complex character development, but from his thought-provoking ideas, most of which were based on scientific fact.
Perhaps his most unexpected plot twist came in his sequel to 2001, called 2010. Towards the end of the novel, the mysterious black monoliths of an advanced and meddlesome race began infesting and multiplying in the atmosphere of Jupiter, adding mass to the big planet in a matter of days. As its mass grew, the planet shrunk, then finally collapsed and ignited as a new star (I won’t tell you why this happened. You can read the book or see the movie if you’re interested; as usual, the book is better).
Here’s a view from the movie of 2010 that shows the collapse of Jupiter into a star. The key moment happens at 3:15 into this clip.
Clarke’s idea is almost entirely accurate. As it happens, Jupiter is not just the largest planet by size in the solar system. Given its composition, structure, and mass, it’s as large as it can possibly be.
If Jupiter was less massive, it would be smaller like Saturn or Uranus. But if you could add 2x, 5x, 10x or more mass to Jupiter, perhaps with the help of Clarke’s magic monoliths, the planet would not grow larger. It would shrink. (Don’t you wish your waistline worked like that).
As it is now, the dense core of Jupiter generates heat which radiates into space. If the planet’s mass increased by 50x to 60x, the core would get hotter and denser until the planet turned into a “brown dwarf”, a type of failed star. At that mass, the planet would start to grow again back to its present size. In fact, all brown dwarfs, whether 10x or 60x Jupiter’s mass, all have roughly the same diameter as Jupiter.
If Jupiter grew to 75x its current mass, its core would get hot enough to fuse hydrogen into helium in its core. It would become small main sequence red dwarf star, and burn steadily for billions of years. Though even then, its diameter as a red dwarf star would only be 30% larger than the planet’s current diameter.
By comparison, our sun, which is bigger than most stars, has a diameter 10x that of Jupiter, but a mass 1000x as great.
You’ll find Jupiter this month in the constellation Capricorn, hovering fat and bright in the southern sky at an impressive magnitude -2.6. It’s the brightest object in the southern night sky, save the moon. Aside from Jupiter, Capricorn has quite a few sights for binoculars and small telescopes, including a lovely double star just west of the biggest planet (see page 59-60 of Stargazing for Beginners for more about what to see in this part of the sky this time of year).
Next time, we’ll give you a few tips on how to observe Jupiter. No monoliths required.
