We know that the earth's chances to be hit by a blackhole before the death of the sun were practically 0 but how do the odds change after the discovery of the unicorn?

About the unicorn: https://www.usatoday.com/story/news/nation/2021/04/23/black-hole-dubbed-the-unicorn-may-smallest-one-discovered-yet/7349740002/

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    $\begingroup$ Note that The Unicorn has not been confirmed to be a black hole. It's not easy to verify that a candidate BH is actually a BH. $\endgroup$
    – PM 2Ring
    Apr 28, 2021 at 20:00

2 Answers 2


The odds haven't changed. It is already known or assumed that high mass stars leave behind black holes. The number of black holes in the Galaxy is likely to be outnumbered by normal stars by at least a factor of 1000, because of the steep decline in the initial mass function at larger masses.

A recent survey of nearby stars suggest none were or will be any closer than about 20,000 au from the Sun over millions of years (Bailer Jones et al. 2018). The close encounter rate is about 20 per million years within 1 pc and scales quadratically with distance. So, the encounter rate with a normal star within 100 au (close enough to affect the inner solar system) would be 1 every 200 billion years. A close encounter with a black hole would be about a 1000 times rarer.


I confess I have some difficulty with calculating the odds, but I think a comparison about right can be made, at least within a few orders of magnitude.

I start with the following source of black holes (BHs).


First, the Unicorn may no longer be the closest BH. The following is closer.

Designation "HR6819Ab". Its mass is >5 SMs (Sun masses), compared with the Unicorn's 3 SM. It's distance from Earth is approx. 1170 ly, compared with Unicorn's 1500 ly.

Second, before one can calculate "how do the odds change after the discovery of the unicorn", one needs to decide which BHs in the list are to be included in the calculation, since many of the entries listed are speculative candidates. If we ignore all known BHs further away from earth than "4U 1543-475A" (those not in the cited list), then it becomes necessary to choose the BHs in the list we want to consider as real BHs rather than just possibly BHs.

I suggest that before I make a calculation, you should choose the BHs in the list you want to include in the calculation.

Please excuse my original errors regarding mass units which I have now corrected. My thanks to @Triatticus @Kosm and @PM 2Ring.

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    $\begingroup$ The Unicorn is three solar masses not three earth masses. $\endgroup$
    – Triatticus
    Apr 28, 2021 at 17:46
  • $\begingroup$ M with circle and dot stands for solar mass, not Earth mass. $\endgroup$
    – Kosm
    Apr 28, 2021 at 19:06

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