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.

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

  1. CS says:

    Referring to the scenario you first mentioned (a Jupiter-like planet becoming a small star), are there any known star systems with a small star and planets orbiting a larger star? Is that thought to be possible? I think a planet orbiting a binary star system was recently detected, but that doesn’t seem like quite the same thing. Thanks.

  2. Fancy Monkey says:

    It seems a “Sun-Jupiter” would pull us out of orbit. Very slowly at first but the feedback loop should build on it’s self. And beyond that pulling Mars or maybe one of Saturn’s moons into a Earth crossing orbit. Or maybe Jupiter starts to fall in towards the Sun.

  3. Dan says:

    In the early days of nuclear weapons, there was an eventually unfounded conjecture that a hydrogen bomb might ignite the Earth’s atmosphere. I realize the issue of insufficient mass of Jupiter, but could a nuclear chain reaction (as opposed to the thermal heating of asteroid collisions, such as Shoemaker-Levy 9) ‘re-spark’ that conjecture. The moons of Jupiter could be close enough to benefit or even flourish from even a weak star, especially if we dig up some mysterious, flat-topped obelisk on the Moon. There is probably a down side I don’t see; reading at night would be cool and there could be a decrease in crime… though ‘light pollution’ would increase. -Dan (“armchair physicist”)

  4. Princess Azula says:

    Most of the energy released in a nuclear bomb is heat. There is simply not enough energy to create a sustained chain reaction in a nuclear bomb. When calculations involving the ignition of earth’s atmosphere where reviewed, it was found that a mistake had been made, there was no chance of that happening.

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  7. Internecivus says:

    I used the Universe Sandbox to randomly make Jupiter a star. It turns out, things don’t go so well for us.

  8. Cd says:

    So we all know mass X pressure(impact)X large amounts of hydrogen can make fusion.. As we have determined take huge amount beyond possibilities for this to happen to Jupiter.. Although there is another version implode? If over 80% of Jupiter’s atmosphere is hydrogen, we all know hydrogen don’t need much to explode except oxygen.( correct?).. Now let’s say it does ignite but don’t burn just consumes itself..would or could this create a gravity inity (blackhole) Jupiter’s mass is big enough and its rotation fast enough to cause a gravity ripple ?

  9. Cd says:

    One more thing to take into consideration.. If Jupiter was big enough to become a star its gravity pull would already be disrupting the magnetic pull between planet and our sun.

  10. Sean says:

    That was an awesome question and and even awesomer answer. HOWEVER, I disagree with your central conclusion that the addition of an object 80 times brighter than the moon would not cause negative impact on life. 80 times brighter means that there would be no night at all for long periods throughout the year. 68% of mammals are nocturnal. And that’s just mammals. These animals would fail to hide or fail to hunt and would certainly die in unusual numbers. The ecological chain reaction of negative impact of 68% of all mammals (not counting nocturnal fish, insects, reptiles, etc.) would be massive. On the other end of the spectrum, some plants and some phytoplankton would suddenly be able grow without interruption with an object 80 times brighter than the moon shining on them all night long night. The mix of plants and sea life would change almost immediately. As go plants and plankton, go herbivors go predators,…So, I disagree, the world would change dramatically and many/all nocturnal species would struggle. Cloud formation is also very impacted by day / night cycles. If night was 80 time brighter, clouds might well form differently. Clouds = rain.

  11. Angeles says:

    But Sean wouldn’t mean that all things would be different in beginning, at the birth of the solar system? Nocturnal species and everything would have evolved to adapt to that. We’d only know of things being the way you are describing instead of what we know now. I believe things on earth would be totally different from the formation of the solar system in your point of view.

  12. Sonny says:

    These questions, however entertaining, are extreme hypotheticals. It is very difficult to ponder these questions because of the incomprehensibly massive scale of the solar system. Jupiter has as much volume as 1000 earths and about as much mass as 300 earths. Jupiter is so large, it is estimated that it comprises approx 3/4 of the rotating mass of the solar system (excluding the sun). Yet the sun itself consists of aproximately 99% of the total mass of the entire solar system. Jupiter is no more likely to suddenly begin fusion like a star than earth’s moon. Suffice to say that the solar system as it appears now is extremely stable, and has been for a long time.

  13. Les says:

    This is totally intriguing. The first part I think the effect is difficult to say our orbit and jupiter’s around the sun differ so sometimes we would be affected with having no night others having a 2 sun during the day. I think things would start to evolve differently to take advantage of this and new species would come. It would be aw inspiring and scary. Think how humans might change eyes to accept the red light or a film over the eye that only is used the time of the year that the red light is there. some many other things wow it is just mind boggling!!!! great conversation. Thank you for the thoughts!!!!

  14. james gellinger says:

    How can there be stars found right now only 1 and a 1/2 times the size of Jupiter but it would take 80 Jupiter’s to equal a Sun maybe it’s compacted mass but it still don’t make since

  15. JAMES A MILLER says:

    A red star 80 times brighter than the moon.
    we would have to change the color of railroad crossing lights, traffic stop lights, brake lights.
    No one would be able to see those red lights at night, with such a bright red star.

  16. Spencer Shepard says:

    James Gellinger, you’ve already answered your own question! (“maybe it’s compacted mass”)

    Large brown dwarfs and small stars can be as much as 200 times denser than Jupiter! As a result, the smallest stars are only about 2x the diameter of Jupiter, despite being 100 times more massive.

    James A Miller, it does not seem like the light from a small red dwarf would make much difference. The sun is about 400,000x brighter than the moon, so the red dwarf would still be 5,000x dimmer than the sun.

  17. Carson says:

    Wouldn’t this scenario put a good tug on the Earth, Mars, and the objects in the asteroid belt giving highly (relative to what they are now I mean) elliptical orbit? Wouldn’t this effect the amount of energy we get from the Sun and drastically destabilize the climate? I understand that you said that Jupiter is about 4 AU(I’m rounding these numbers) from us, but that cannot be counted as an out. Neptune was discovered mathematically, not visually because we noticed an irregular tug on Uranus’ orbit. That’s a distance of roughly 11 AU. Earth to jupiter is only 4 AU and in this scenario (or even if you don’t count the scenario of horribly named Jupiter star) the star would be 100 times more massive than Jupiter is which is much much more mass than what’s in Neptune and is more than twice as close.

    Even slight variations in things like orbit path, axis tilt, and spin strongly dictate the course of interglacial periods and other aspects of climate, as well as gravity effects different geological aspects of rocky astronomical bodies. So I can see how it might not effect life if it formed, but if we were part of a binary system the Earth would be a drastically different place than it is currently and I can only imagine the chaos this would cause if this happened some how over a short (cosmologically speaking) timespan. All of this is up in the air assuming that this system would create the right conditions for life to form in the first place.

    I don’t even know where to begin with the math on this, but all the planets effect their neighbors, if even by a bit.

    BTW I stumbled on this fact checking a junk science page, and seriously thank you for posting this. Fascinating topic for postulating and was a great break. I’m book marking you!

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  19. Gerard Ammerlaan says:

    I wonder what would happen if Jupiter became a star. Jupiter’s mass would have to increase dramatically, which might have implications for our planet’s orbit around the sun.

    I wonder if and how much our planet’s orbit would alter in that case….

  20. Princess Azula says:

    Earth’s orbit would not be changed if Jupiter were a small star (the mass would be 100 current Jupiter masses). Jupiter and the Sun would orbit each other around a barrycenter (a common center of mass) this would be about one-half of an AU (AU=earth-sun distance) from the Sun. Earth would only be fine if Jupiter and the sun orbited this barrycenter at Jupiter’s current low eccentricity. Mars however would be kicked out as would most asteroids. Saturn’s orbit would also be unstable. This zone of instability would be from just inside Mar’s current orbit (1.6 AU) to 16 AU (well outside Saturn’s orbit). This could only be possible if the Solar system was created this way with Jupiter and the Sun orbiting the barrycenter at very low eccentricities if a small star entered our system and crashed into Jupiter and entered into orbit around the Sun it is likely that the eccentricity would be too high and that earth would be thrown out of the Solar System.

  21. monkeyangel says:

    I don’t know what it means that well. Because I’m more of a math girl while i have to learn this for science. But what would happen if we were on Jupiter when it was star? Would we die?

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