I read recently that metal-rich planetary systems around large (>5 solar masses) O- and B-type stars can form enormous solid planets (potentially bigger than Jupiter) relatively quickly, photoevaporating their atmospheres to ensure that they don't become gas giants. Unfortunately these types of stars tend to have lifespans of 50 million years or less; not enough time for really interesting planets to arise before the inevitable supernova.

How can I have a high-metallicity system older than 50 million years that has an O- or B-type star with at least 5 solar masses which is expected to last for at least another 10 million years?

I'd like to get as long of a lifespan as possible while still keeping the star massive enough to have the UV radiation and solar winds to form massive solid planets in a reasonable timeframe. Habitability isn't a concern, but I'd like at least a couple of the inner-system solid planets to be able to maintain an atmosphere of at least a couple bars with average surface temperatures under 1000 °K.

Unless there are ways to make a star like this last for quite a bit longer than is listed on the lifespan chart, my best bet is probably to have a non-blue star transition into a blue star.

A couple of theories based on my research:

  • Blue stragglers can form in stellar clusters via stellar collision or mass-transfer from a binary companion. This results in a star that has 2-3 times as much mass as other stars in its cluster, potentially allowing for a star to form planets normally and then grow to (hopefully) over 5 solar masses, turning its inner gas giants into chthonian planets.
  • Horizontal-branch blue giants can pass from a red giant phase to a blue giant phase before moving on to the asymptotic giant branch (which caps the star at 10 solar masses, precluding it from being O-type). Since blue giants tend to have upwards of 7 solar masses, this definitely meets my size requirement, but I haven't been able to find out how long the period between the formation of the star and the end of the blue giant phase would be.
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    $\begingroup$ When I saw the question title, my initial thought was "blue stars don't get old", but it turns out you already did some research on this. I don't have an answer for you, but kudos for that! We need more people to do their research before throwing random questions at the Worldbuilding SE community. $\endgroup$
    – user
    May 12, 2016 at 16:23

2 Answers 2


Why not have a large rocky planet form around a blue giant, then ejected from the system, and have it attend another star in the same birth cluster for a while? The advantage of the short life is that the star will still be in a dense starforming region where hundreds or thousands of stars are forming from a cloud lightyears across, all around the same time.

We know that many protoplanets are lost, ejected or falling into the star.

Now, a massive solid object like this superrock might have a significant effect on the formation dynamics as it arrives right as in-situ planets are starting to form.

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    $\begingroup$ Another interesting effect you could get from this is that this planet's orbit around the star would highly likely be in a completely different plane than the remaining protoplanetary disk. This has two significant implications: (1) it will be passing through the protoplanetary disk twice in each orbit, while the material in the disk is orbiting the star (so early on, you get "holes" more than a clean sweep), and (2) if any planets do form around the star, those will orbit in a potentially very different plane (think our solar system vs Pluto, but possibly even more extreme). $\endgroup$
    – user
    May 14, 2016 at 19:36
  • $\begingroup$ I don't think it could compete with the total angular momentum of the disc. But sweeping out material in a different plane might have very lasting effects upon subsequent steps. $\endgroup$
    – JDługosz
    May 15, 2016 at 10:25
  • $\begingroup$ I didn't mean that it would. (Unfortunately, I ran out of comment space.) Rather, the odds that a captured rogue planet would be in the same plane, perhaps to within a few degrees, as the protoplanetary disk (assuming one exists) around the star seem tiny (but of course non-negligible). From this it follows that a captured rogue planet would be passing through, not orbiting in plane of, the disk. The implications I mentioned earlier follow logically from that. I agree that a significantly off-plane orbit probably wouldn't have much direct effect except where the planet passes through the disk. $\endgroup$
    – user
    May 15, 2016 at 11:54
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    $\begingroup$ I think it's a neat idea and can account for some pretty exotic systems. Looking at the SETI Coloquium seminars over the last year or two that detail the latest findings on planetary system formation, throw a monkeywrench into the works with this big rock. If not "super rocks", but sizable planets and brown dwarfs ought to be around from neighboring systems from time to time in star formation. So I wonder we've seen such things already in real life? $\endgroup$
    – JDługosz
    May 15, 2016 at 18:13
  • $\begingroup$ Would it be at all plausible for there to be a couple such captured planets (possibly on different orbital planes from each other even)? As I mentioned in the question, I'd like to have at least a couple non-primordial planets in the system. Also, would you be able to direct me to any sources on what captured rogue bodies would do to the formation of a new system? $\endgroup$
    – emo bob
    May 15, 2016 at 19:33

Close barycenter

At the moment I don't have time for doing the full research, but I remember something that might be worth a look for you: close binary systems.

How does this work? Some stars are said to whirl around their barycenter fairly close... close enough to have the bigger of the two suck up the material of the smaller one.

So if both stars start their life at the mass range of two to four solar masses, you get the time you need to develop "something" in that system. Sadly I cannot offer any kind of formula or even extrapolation that tells you how long it would take for the slightly bigger star to "drain" his fellow binary star.

Its like... having on with 5 solar masses (gives him what? 500 million to one billion years lifetime?) and a smaller one with... say... two to three masses. I think that this will go bada-boom way before all mass had been transfered, but in the meantime your 5 masses start might grow up to 6 or 7 masses.

To be honest, thats a pretty quick brainstorm happening at my side. Chances are low (but are there), that my knowledge of this is outdated and stars wont circle around close enough to make this happen after all.

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    $\begingroup$ This sounds like the setup for a Type II a Supernova. The star take so long to suck up each other's mass that it dies, turns into a white dwarf and goes into electron degeneracy. I don't think that's what you want. $\endgroup$
    – Aron
    May 13, 2016 at 10:20
  • $\begingroup$ @Aron That's not how Type Ia (which is what I assume you meant; there is not Type IIa) supernovae work. The other body needs to already be a degenerate body, like a white dwarf; mass transfer between two stars of 2-4 solar masses will only lead to a subgiant and a star still on the main sequence (see the Algol paradox). (The reason degeneracy is important is that the equation of state of a white dwarf is not dependent on temperature, and so it will not expand if heated, unlike a normal star, but will detonate.) $\endgroup$
    – HDE 226868
    May 17, 2016 at 21:45

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