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Comment by vld_chk | Hacker News Reader

Whenever I read about Planet Nine search, I have a very naive (as a non-physicist) childish question: if we detect anomaly in our Solar System just less than a decade ago and can’t find anything visual which explains it, assuming there is giant planet/piece of ice floating somewhere at the edge of Solar System, how we are sure that there are not a lot of such objects and real space is not as “empty” as we think it is? Simply, what is the probability that let’s say space between us and Alpha Centaurs is not filled with objects like this? Invisible and leaving a tiny gravitational trace at the edge of our ability to detect it?

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The simple answer is that there are actually very large numbers of objects in the space between stars that aren't detectable. Including, certainly, planet-sized objects that formed in stellar environments, escaped, and now fly freely in between.

https://en.wikipedia.org/wiki/Rogue_planet

The so called Rogue Planets.

There's a nice video from Kurzgesagt on the topic: https://youtu.be/M7CkdB5z9PY?si=0BwFwotoHi8PxL1f

Indeed - and the Oort cloud itself extends to about 1.5 light-years, which is a substantial fraction of the ~ 4 light-year distance to our nearest neighboring stars.

That's even more so substantial if the nearest neighboring star also has a comparable oort cloud. In that case a mere 1ly of interstellar space would separate the two systems

If there is so many of these materials not necessarily planet, can we have this partially dark matter?

Very good question. As a non-astronomer, my guess is that we don’t observe a lot of transient black outs for example of the milky way. Which would happen if there’s a lot of these around.

That gives you an upper bound of how many objects like that exist.

Microlensing events! (to be slightly more precise)

https://en.wikipedia.org/wiki/Rogue_planet#Microlensing

- "They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two Jupiter-mass rogue planets for every star in the Milky Way.[26][27][28] One study suggested a much larger number, up to 100,000 times more rogue planets than stars in the Milky Way, though this study encompassed hypothetical objects much smaller than Jupiter.[29]"

And "MACHOs":

https://en.wikipedia.org/wiki/Massive_compact_halo_object

In the 1990s I went to a talk which suggested that "WIMPs" (Weakly Interacting Massive Particles aka supersymmetric particles that would give rise to dark matter) was an acronym for "Well It Might be Physics" while MACHOs was "Maybe Astrophysics Can Help Out".

The upper bound on MACHOs has looong since eliminated them as the explanation for all of dark matter though...

...or an upper bound of how big they might be. For all we know, the galaxy might be full of really dark bowling ball sized objects. We'd have no way of observing them, right?

What if dark matter is primordial bowling balls!

So these are called "snowballs" and are excluded as dark matter candidates because we would then see a much larger number of extra-solar asteroids/meteorites. Cf. e.g. section 2 of the reference below.

https://web.archive.org/web/20180723032406/http://iopscience...

I love how every time someone comes up with some random notion, a scientist can point at a chart and say “we’ve thought of that already and excluded it with experiment CoolName.”

Really dark bowling balls would still exhibit blackbody radiation

The gravitational effect of a lot of spread out mass outside the Oort cloud (or indeed, a lot more mass in the Oort cloud) would have a different effect on the orbits of the planets than a single planet that we had not found yet.

So for some known configuration of orbits of the known planets, there are a limited number of solutions (in terms of mass, inclination, eccentricity, etc.) that you could add to the gravitational interactions of the solar system and still have the current orbits we observe. That gives a reasonable guess about what (another planet) and where (in the sky) to look for to explain anomalies.

But with distant objects that don't emit their own light, even having a good guess of a bunch of the orbital parameters doesn't mean it is easy to find, because you don't have much sunlight reflecting off of it, and the chances it will occlude something else brighter (like a background star) are needle-in-a-haystack level.

Even _if_ you knew the _exact_ orbit you were looking for, there are 360 degrees of sky to search for a tiny, dark object, because you don't necessarily know where in the orbit the object currently is.

> how we are sure that there are not a lot of such objects and real space is not as “empty” as we think it is?

There could be lots of such objects without meaningfully denting how empty we think space is.

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