Q: How close is Jupiter to being a star? What would happen to us if it were?

The original question was: I have heard Jupiter referred to as a failed star.  That if the cosmic chaos of the early solar system had worked out a little different, and Jupiter had gotten a bit more mass, it might have been able to light the fusion engine and become a star.

Two questions.

How close was Jupiter to becoming a star?

If something really big slammed into Jupiter today, could it trigger nuclear fusion?

Ok and a third question.  If Jupiter did in fact get slammed with something big enough to trigger nuclear fusion, and it became a star, how long would it take to substantially alter the ability for earth to sustain life as we know it?

Physicist: That is a really cool question!

I heard the same thing a while ago, but Jupiter is a long way from being a star.  That estimate was based on some old nuclear physics (like 1980’s old).  By being awesome, and building neutrino detectors and big computers, we’ve managed to refine our understanding of stellar fusion a lot in the last few decades.

Although the material involved (how much hydrogen, how much helium, etc.) can change the details, most physicists (who work on this stuff) estimate that you’d need at least 75-85 Jupiter masses to get fusion started.  By the time a planet is that large the lines between planet, brown dwarf (failed star), and star get a little fuzzy.

So, for Jupiter to become a star you’d need to slam so much additional mass into it, that it would be more like Jupiter slamming into the additional mass.

If you were to replace Jupiter with the smallest possible star it would have very little impact here on Earth.

There’s some debate over which star is the smallest star (seen so far).  OGLE-TR-122b, Gliese 623b, and AB Doradus C are among the top contenders (why is every other culture better at naming stars?), and all weigh in around 100 Jupiters.  They are estimated to be no more than 1/300th, 1/60,000th, and 1/1,000th as bright as the Sun respectively.  So, lets say that Jupiter suddenly became “OGLupiter” (replaced by OGLE-TR-122b, the brightest of the bunch, and then given the worst possible name).  It would be a hundred times more massive, 20% bigger, a hell of a lot denser, and about 0.3% as bright as the Sun.

At it’s closest Jupiter is still about 4 times farther away from us than the Sun, so OGLupiter would increase the total energy we receive by no more than about 1 part in 5 thousand (about 0.02%).  This, by the way, is much smaller than the 6.5% yearly variation we get because of the eccentricity of Earth’s orbit (moving closer and farther away from the Sun over the course of a year).  There would be effectively zero impact on Earth’s life.

There are examples of creatures on Earth that use the moon for navigation, so maybe things would eventually evolve to use OGLupiter for navigation or timing or something.  But it’s very unlikely that anything would die.

OGLupiter would be around 80 times brighter than a full moon at its brightest, so for a good chunk of every year, you’d be able to read clearly at night.  It would be very distinctively red (being substantially colder than the Sun), and it would be clearly visible even during the day.