Why Can’t we see the Oort Cloud?

The Oort Cloud is a mysterious entity. Located on the outskirts of the Solar System, this hypothetical region is probably the source of the long-period comets that occasionally pass through the inner planets’ orbits. The strange thing about these comets is that they have orbits inclined at pretty much any angle from the ecliptic which suggests their source isn’t a belt confined to the ecliptic plane (like the asteroid belt or Kuiper belt). Therefore, their proposed source is a cloud, acting like a shell, surrounding the Solar System.

OK, so we think the Oort Cloud is out there, and there is a lot of evidence supporting this, but why can’t we see the Oort Cloud objects? After all, the Hubble Space Telescope routinely images deep space objects like stars, galaxies and clusters, why can’t we use it to see embryonic comets within our own stellar neighbourhood?

This question recently popped up as a response to my recent Ten Mysteries of the Solar System article over at the Universe Today. So why can’t Hubble be used to observe objects in the Oort Cloud? If it could, surely we’ll then have observational evidence as to the existence (or not) of this mysterious region of space.

Fortuitously, in response to the Moon landing conspiracy, Phil Plait over at Bad Astronomy, approached a similar question; not about the Oort Cloud, but about the lunar module. Those who believe the lunar landings did not happen use all kinds of “proofs” that the Apollo missions were actually staged in a Hollywood sound stage. One such example is the erroneous statement: Obviously the lunar landings didn’t happen, we would have used the Hubble Space Telescope to see the Lunar Module (LEM) descent stages left behind by now… So Phil explains the resolving power of Hubble and showed that any object on the Moon would have to be at least 200 metres wide to be seen (the stuff left over after the landings were a maximum of 4 metres diameter, and so cannot be seen). This may seem surprising, but you have to remember, the galaxies and nebulae resolved by Hubble are huge objects, even the fine-scale wisps of gas and dust measure light years across.

So back to our Oort Cloud objects. Firstly, I’m no practical astronomer, I’m an astrophysicist (I know how the cosmos works – apparently – I just don’t know how to see it!), so I’ll use our Bad Astronomer‘s formula on the resolving power of Hubble. If we assume Hubble can resolve objects that are 0.1 arc seconds diameter or larger, we have a starting point to see how big an Oort Cloud object would need to be to be observed. But first, let’s see if an existing Oort Cloud comet can be observed by Hubble.

Using the equation: (d / D) × c = φ

where d is the diameter of the Oort Cloud comet (some estimates put this number at an upper limit of 300 km for the diameter of a cometary nucleus), D is the distance from the Oort Cloud to Hubble (0.3 light years, or 3×10¹⁵ metres – distance at which it is theorized there is the highest density of Oort Cloud objects), c is a constant (c = 206265) and φ is the telescope resolution.

So what resolution do we need to image an Oort Cloud object, 300 km in diameter, from 0.3 light years away? If we plug in the numbers we get:

φ = 2.06×10^-5 = 0.00002 arc seconds

The resolving power of Hubble is 0.1 arc seconds, and is therefore useless at detecting anything below this angular size; Oort Cloud comets (although pretty big at an upper limit of 300 km) simply cannot be observed by the world’s most advanced space-based optical observatory.

But how big would an Oort Cloud observing telescope have to be to resolve a cometary nucleus 300 km wide at a distance of 0.3 light years away? Using the simple relationship R = 11.6 / w, where R is the resolving power (R = 0.00002/2; the reason for halving our resolving power is given by Phil), and w is the width of the telescope mirror, we rearrange to get:

w = 11.6 / 0.00001 = 1.16×10⁶ cm = 11.6 km

As you can see, such a telescope would be huge. Unless there is some large baseline optical telescope that can use an array of observatories to make up for the 11.6 km-wide telescope mirror, we simply cannot observe Oort Cloud comets. In fact, the smallest object Hubble can resolve at a distance of 0.3 light years is nearly 730,000 km diameter (roughly half the diameter of the Sun!).

It would seem that the Oort Cloud objects are as inconsequential as grains of dust floating in our atmosphere when we peer through our telescopes and look at the Moon. We know the dust must be there, but we see straight through it as if it wasn’t…

Sources: Bad Astronomy and Universe Today