Stellar Flybys: New Way To Life’s Extinction


How fragile is life?


Many disasters can strike a biosphere.

A famous “equation” the Drake equation, is meant to estimate the probability of life in the galaxy. It overlooked many factors. One of them is interstellar near-misses. It is not a question of stars colliding, but of the disruption of solar systems.

A star zoomed through our Solar System just 70,000 years ago, astronomers have just discovered.

Scholz's System: I Zoomed By, Therefore I Scare

Scholz’s System: I Zoomed By, Therefore I Scare

No other star is known to have approached this close to us.

The international team of astronomers found that the intruder came five times closer than our current nearest neighbour – Proxima Centauri (at .8 light year whereas Proxima Centauri is 4.2 light year away).

The star, a red dwarf known as Scholz’s star, cruised through the outer Oort Cloud, a region not as much stuffed with comets as the inner Oort Cloud. Scholz’s Star is 8% of the mass of the Sun, and it is accompanied by a brown dwarf which is nearly as massive (6% of Sun mass). It goes without saying that going through the comet cloud would have adverse consequences for advanced life on Earth. But worse could happen.

(Brown dwarves just miss the mass necessary mass to have enough heat and pressure to get fusion going in their cores).

Something struck me when extrasolar planets were discovered. How frequent were the super-Jupiters grazing their home stars? Of course, there was a detection bias (with the technology used by the French Corot, and the more recent USA Kepler, the probability to detect a super-Jupiter close by was overwhelming).

However, a question loomed: how do you get a gas giant that was obviously formed far from the home star, so close to the home star? One could imagine a cloud breaking the planet, but that makes little sense (as cloud and planet have similar angular momentum, and the planet would suck the cloud).

What’s left?

Collisions. Or more exactly, near-misses.

Previous work suggests that flybys within 0.25 pc occur infrequently (~0.1 Myr−1).

Sedna, a dwarf planet, whose orbit varies between 70 (seventy) and 1,000 (thousand) Astronomical Units (AU, the distance between Sun and Earth). Maybe a remnant from a near collision (torn from a Red Dwarf clutches).

In any case, Scholz’s star came within 52,000 AUs of the Sun. And it passed very fast (the slower the pass, the greater the disruption).

If it had passed within 100 AUs, the disruption would have been considerable (Neptune is at 30 AUs; at 90 AUs, the pull from a passing star, in the worst case would 1/10 of that of the Sun, and would make the orbit of Neptune ellipse, significantly eccentric)

So what happens if a gas giant gets a severely eccentric orbit? Well, it can cut through the others’ orbits, and the whole system becomes unstable. A few large collisions and near misses later, one could get some gas giants to graze their suns, as observed.

As usual, I just suggest the idea. Others can figure out the details, program their computers, and check… ;-).

Let’s make a little computation. Suppose that the probability of a star coming within 52,000 AUs was once every 100,000 years (a probability tellingly estimated BEFORE it came to be known Scholtz’s star zoomed by).

Let’s consider the disruption radius to be 100 AUs. The probable number of near-misses disrupting the inner system during the past extent of the Solar System would then be:

1/25,000)x(10^4)x5 = 2.5.

That’s quite a bit… And, now that we know about Scholz’s Star’s recent flyby, no doubt that the probability of near misses will skyrocket. All the more as the Sun is presently in the pretty empty zone, the Local Bubble, 300 light years across.

I have explained in the pat that life depended not just upon having a planet in the Habitable Zone (the Water Zone), but also in the Radioactive Belt.

Many are the causes of disasters on the way to advanced life, as I have enumerated in:

Being lucky with stellar flybys is another factor to consider: a nudge to the outer gas giants, and it’s curtains for advanced life. Bacteria don’t count. I am suggesting that, in star systems long established, star near-misses is a much serious problem than rogue asteroids or comets.

Patrice Ayme’


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11 Responses to “Stellar Flybys: New Way To Life’s Extinction”

  1. gmax Says:

    You left rogue planets out of your disaster parade… Although I guess that’s what mentioning SEDNA was all about. If Sedna were a gas giant. And orbited closer… BOOOM!

    Those Harvard guys talk about a near-miss with the star of which Sedna was a satellite, four billion years ago

    • Patrice Ayme Says:

      Rogue planets, except from very great unluck, are more a symptom than a worry. They show the instability of this whole celestial mechanics.

      BTW, another explanation for Sedna’s orbit is the presence of a large planet out there…

  2. ianmillerblog Says:

    The simplest explanation for so many gas giants close to their star is, I believe, as follows. The stellar accretion disk takes a My to form the star, then the remnants hang around for ~ 1 -20 My, depending on the star, chance, whatever, at which time the star sends out massive bursts of energy and clears the disk. Planetary accretion stops then. The star LkCa 15 has a planet about 3 times further from its star (about the same size as Sol) as Jupiter, and it is about 5 times more massive. To me, that means our star must have been one of the very quick ones to clear its disk. (Note that at 15 A.U., according to standard theory, it should not have formed!!)

    If the disk lasts longer, the planets keep accreting as the gas/dust continues heading towards the star, albeit at a rate about 4 orders of magnitude less than in the first My, so the planets simply keep growing. If they get big enough, and they are close enough together, they start playing gravitational billiards, throwing one out, say, and sending the other on an eccentric orbit inwards. That explains why so many orbits are highly eccentric. If the orbit is sufficiently eccentric or the semimajor axis is small enough, tidal interactions with the star circularise the orbit. That, by the way, is what I believe, but I did not originate it.

    For anyone interested, this and a number of what some would call heresies are in my ebook, Planetary Formation and Biogenesis.

    • Patrice Ayme Says:

      Dear Ian: Your theory is interesting. What it boils down to, it seems to me, is that you envision very dense star systems, with large planets close by. I agree that then, thanks to collisions some will end up close to the star.

      So the question is: how many systems are born as ultra dense clouds? If a large percentage of them are not, then the mechanism I propose may not be so unlikely. All the more since many stars are born in star-dense zones (lots of star near-misses afterwards… Especially on the timescales you suggest… A few million years).

      In any case, it would have an impact on life: a planetary system with haphazard planets will be hostile to life. I am just suggesting that, in star system long established, star near-misses would be a much more serious problem than rogue asteroids, comets and the like.

      • ianmillerblog Says:

        The system doesn’t have to be extremely dense,although it depends on what you mean by “dense”. The concept is there dis a continuous flow of gas and dust towards the star, and the planets collect what goes by in their gravitational zone. Rather than dense, the gas has to continue flowing for a long time. Also, if you mean by dense, compact, then yes, compact helps because planets perforce being closer together have greater gravitational interactions. Basically, in my theory systems become a lot more compact if the star is smaller, and observations of red dwarfs are in accord with this.

        • Patrice Ayme Says:

          Agreed with Red Dwarf system (red systems?) being denser. Everything smaller there. Full of flares, I have changed my mind about them for habitability. BTW, I wanted to exonerate Nemtsov some more… Have to run now…

  3. dominique deux Says:

    Just love it when you write about science I can more or less follow.

    • Patrice Ayme Says:

      And I love it when you love it! I have more to say on that general subject, habitability, that surfaced as recently as 2015…
      BTW, the non-understanding of Quantum Physics is a scientific problem, not just a pedagogical one!

      • Paul Handover Says:

        Agree with Dominique especially the “more or less” part of his sentence! Although I can’t resist lowering the tone of things by passing on that well-known statistic that 6 out of 3 scientists are dyslexic.

        • Patrice Ayme Says:

          Dear Paul: This was written fast, so it’s far from perfect, I actually forgot to indicate the Red Dwarf’s speed, 80 kilometers per second (an important point, because the faster, the less of a gravitational influence). So you can just cut and paste what seems obscure, or a typo, and put it in a comment, ;-)!

  4. Hawaiian Savagery & Planet Nine | Patrice Ayme's Thoughts Says:

    […] Stellar Flybys: New Way To Life’s Extinction […]

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