Close Encounters of the Stellar Kind

In Stars by Brian Koberlein9 Comments

A star known as HIP 85605 is on a collision course with our solar system. Not a collision course as in Armageddon, or When Worlds Collide, but rather on a path to come within 0.04 parsecs of the Sun. That might seem close, but that would make it more than 8,000 times more distant than Earth at its closest approach (8,000 AU). Distant Eris is only 100 AU by comparison. Given that stellar distances are typically measured in light years, and this encounter would be only 50 light days away, that’s remarkably close by astronomical standards. This close encounter won’t occur for another 400,000 years or so. Currently HIP 85605 is about 16 light years away, so we have time to plan for its visit. But it does raise an interesting question as to how such a close encounter could affect our solar system.

One thing we know for sure is it won’t significantly affect planetary orbits. Through computer simulations of the solar system, we find that orbits of the inner planets remain stable over billions of years even with stellar interactions as close as 0.01 parsecs. The most outer bodies such as Pluto and Eris are less stable, but on the whole planetary orbits are stable against all but nearly direct stellar hits.

Where things get less certain is how a stellar encounter would affect bodies in the Oort cloud. Although we know the Oort cloud exists, we aren’t sure about its size. The typical estimate is that the outer Oort cloud is between 20,000 and 50,000 AU from the Sun. In that case HIP 85605 would actually move through the Oort cloud, which would seriously disrupt the icy bodies within it. Some would be flung out of the solar system forever, while others would be thrown toward the inner solar system. This could cause a dramatic increase in the number of comets crossing Earth’s orbit. This uptick would happen over a million-year time scale, but that’s fairly short on geologic scales. What that might mean for life on Earth isn’t entirely clear.

It’s estimated that about 40 stars have made a “close” encounter with the Oort cloud over the last 20 million years, and it has been proposed that cycles of cometary impacts due to such collisions could have led to climate variation and extinction events on Earth. But the evidence for such a connection isn’t very strong. There isn’t a strong correlation between impact events and extinction events, the great dinosaur impact not withstanding.

Given the long history of life on Earth, it isn’t likely that such close stellar encounters have a huge affect on the solar system and life on Earth.

Paper:  C.A.L. Bailer-Jones. Close encounters of the stellar kind. arXiv:1412.3648 [astro-ph.SR]


  1. Thank you for making my day so much nicer with that information 😀 Earlier in December, The Planetary Society had asked the public where did we want space exploration to take us next? We were asked to reply using the hash tag #SpaceFan and wrote the following short twitter poem.

    “I wanna hitch a ride with a hypervelocity star if any come our way, to some other galaxy if heading that in that direction someday #SpaceFan ”

    So, I don’t know about you, but I’m making plans for the future, at least for future generations. Likewise, I’m sure we could hitch a ride on comets to the outer solar system far sooner than the latter. Perhaps if we could use landing mechanisms for comets that would keep spacecraft stuck to the comet surface, as in a smallish reverse thrust rocket engine system, to enable the craft to anchor itself until it could drill itself a strong hold?

    For the link to the actual Twitter poem click on

    And for a longer version of the poem in the comments and the YouTube video by Emily Lakdawalla which prompted it click on

  2. Not bad, Andrew. I was told by a scientist recently that our Sun could very well become a hypervelocity star once the Milky Way Galaxy merges with Andromeda. However, humanity is not likely to be alive by then; and if we are, we will no doubt bear no resemblance to our current selves. I suppose if we are going to hitch a ride on a planet being towed by such a star, we’re going to have to hitch onto it! 🙂

    1. Thanks Matthew. I agree that our species is still evolving, both genetically and via the extended phenotype that is technology. Though we might not bear any resemblance to our currant selves in the far future, if we evolve to be clever enough, we will be able to perpetuate any species at any period in its evolution whose genome we might have preserved and by then even if that preservation is just purely digital. It might become a norm for really evolved species to propagate entire ecologies of a certain period just for the sake of it like present day zoos, but on a larger scale.

  3. Begs the question, are runaway stars more likely to have intelligent life carried along within their gravitational field than other stars, because if our species has thought of using them for space travel, so would extraterrestrials?

  4. The latest on hyper velocity stars and I quote, “Scientists using the W. M. Keck Observatory and Pan-STARRS1 telescopes on Hawaii have discovered a star that breaks the galactic speed record, traveling with a velocity of about 1,200 kilometers per second or 2.7 million miles per hour. This velocity is so high, the star will escape the gravity of our galaxy. In contrast to the other known unbound stars, the team showed that this compact star was ejected from an extremely tight binary by a thermonuclear supernova explosion. These results will be published in the March 6 issue of Science.”

    Read more at:

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