Does Our Solar System Contain a Primordial Black Hole?

Astronomers think there's an object in the far reaches of our solar system that's six times as massive as Earth. Learn why some think it's a planet and others propose it's a type of black hole in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of iHeart Radio. Hey brain Stuff,
Lauren bog ol Bomb. Here there's something big lurking in
the frozen hinterlands of our Solar System that appears to
be tugging at the tiny worlds beyond the orbit of Neptune.
The object is assumed to be a hypothetical world called
Planet nine, which has an extremely far flung orbit around

(00:23):
the Sun and causes all kinds of gravitational chaos out
there in the dark. But as the search for Planet
nine wears on and astronomers have yet to so much
as get a glimpse of it, some researchers are pondering
what else the object could be. Might it not be
a planet at all? Could it be a primordial black hole?
The Solar System is a big place, and while our

(00:44):
astronomical techniques are rapidly advancing, many tiny bodies in the
outer Solar System have yet to be found. Planet nine
is thought to be a rather more substantial object, however,
with a massive around five to ten Earth masses scooting
around the Sun at an average distance of four to
eight hundred astronomical units or a US, since one AU
is the average distance at which Earth orbits the Sun.

(01:07):
That's ten to twenty times the orbital distance at which
Pluto orbits the Sun, So if it exists, planet nine
takes between ten thousand and twenty thousand years to complete
just one orbit. The possibility of a large world orbiting
the Sun at such a huge distance is captivating. Studies
of other star systems reveal that exoplanets between the masses
of Earth and Neptune are relatively common. Why our Solar

(01:31):
System doesn't contain a world with this mass range is
a puzzle. But if Planet nine really is out there,
it would be a profound historic discovery that would reshape
our understanding of the system of planets that orbit our Sun.
Suffice to say, any planet with an orbit this extreme
would be very difficult to spot, but astronomers are scouring
infrared surveys with hopes of seeing a distant object slowly

(01:53):
crawl across the sky. If it's out there, planet nine
should be emitting infrared radiation entered leaking from the planet
since its formation, but so far, apart from the gravitational
effects of something in the outer Solar System, there's precious
little direct evidence for Planet nine existence. Enter the black
hole hypothesis. In September of twenty nineteen, astronomer's Jacob Schultz

(02:17):
of Durham University and James Unwin of the University of
Illinois Chicago published a new study describing their alternative hypothesis
that the gravitational weirdness in the outermost reaches of the
Solar System isn't being caused by a planet at all. Instead,
they pointed to the potential presence of a primordial black hole,
a theory that's caused a bit of a stir No.

(02:37):
This kind of black hole does not pose a danger
to the rest of the Solar System, it would be
too small for that, but in the distant regions of
our Solar System its impact would be significant. The only
evidence we have for Planet nine existence are the gravitational
effects it's having on trans neptune objects, and black holes
are the most gravitationally endowed objects in the universe. After all,

(02:59):
Primal real black holes are the most ancient kind of
black hole. They're hypothesized to have formed right after the
Big Bang. Density fluctuations in the early universe would have
rapidly formed black holes of all masses. These ancient objects
would have been flung throughout the cosmos, and over time
they would have slowly evaporated via Hawking radiation, smaller ones

(03:19):
popping out of existence first. But let's back up a step.
Why designed to think that some extreme object is out there?
In planet Hunter's Constantine, Batiguon and Mike Brown of Caltech
announced their discovery of a group of very distant trans
Neptunian objects that were all strangely clustered and moving with

(03:40):
similar orbital alignments. Their orbital alignment was also weirdly tilted,
so it appeared they were all being corralled by gravitational
interactions with a larger planetary body. But no other large
planetary body exists in that region, So Batigan and Brown
hypothesized than an as get to be discovered planet was
out there, and so the hunt began. While many theories

(04:03):
of cosmic evolutions suggest that primordial black holes should exist,
we have yet to directly observe one, though there is
some compelling indirect evidence. Take, for example, micro lensing events,
the transient brightening of stars caused by a massive object
passing in front of them, causing a brief brightening via
the curvature of space time, creating a sort of magnifying lens.

(04:25):
Analysis of these micro lensing events suggests that there's a
population of small black holes out there with no other
visible clues except for their gravitational impact on space time.
Schultzon Unwin took a fresh look at the trans Neptune
object peculiarities and simulated what would happen if a black
hole with a mass of between five and ten Earth
masses had an extreme orbit around the Sun. Sure Enough,

(04:47):
their models suggest that a primordial black hole with a
mass within this range would cause orbital perturbances like the
ones already observed in the population of trans Neptune objects.
This could also explain why little optical or infrared observational
evidence for planet nine exists. A primordial black hole would
generate neither signal. In fact, if a black hole is nearby,

(05:09):
it may also be dragging around a cloud of dark
matter that could be generating different types of radiation. The
researchers therefore suggest, based on their findings, that the experimental
program needs to be expanded, including searches for high energy
cosmic rays like X rays and gamma rays coming from
moving sources. While this is an interesting avenue of study,

(05:29):
replacing a hypothetical planet with a hypothetical type of black hole,
maybe over complicating the mystery of planet nine. We spoke
with Mike Brown. He said, could a black hole explain
the gravitational effects we are seeing in the outer Solar System? Absolutely?
All we know is that there is a six earth
mass something out there, and we don't know what the
something is. Brown points out that a planet would be

(05:52):
the most obvious something, but as long as it has
a mass of a few earths, it could be anything.
But the plausibility of it being an anything other than
a planet is extremely low, to say the least. With
tongue in cheek, Brown added, it might be a six
earth mass hamburger or a burrito, but yes, it might
also be a six earth mass black hole. The physics,
of course, don't care one bit what the six earth

(06:14):
masses are made of. You could equally well hypothesize that
every exoplanet that we only detect via the radial velocity
method is a black hole. Is it possible? Yes? Does
it make any sense in the universe? No. While investigating
other gravitational sources of what may be messing with transniptune
objects is good science, it's more likely the planet nine

(06:35):
is a planet and not a primordial black hole. Astronomers
just have to keep searching, and there's a growing consensus
that it will be discovered in the not so distant future.
We'll keep you updated. Today's episode was written by Dr
Ian O'Neill and produced by Tyler Klang. Brain Stuff is
production of I Heart Radio's How Stuff Works. For more

(06:57):
on this and lots of other topics that are surprisingly
dense for their size, visit our home planet, how stuff
Works dot com. And for more podcasts from my heart Radio,
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