Why Can’t we see the Oort Cloud?

Hubble Space Telescope observation of Polaris – looking through the Oort Cloud, but not resolving any comets (Hubble)

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

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16 responses to “Why Can’t we see the Oort Cloud?

  1. Pingback: The Daily Links - August 27th « The Four Part Land

  2. Pingback: Uncategorized world news » Blog Archive » Why Can’t We See The Oort Cloud?

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  6. sure and when we go back to the moon then I would believe. but alas their constellation program got cancelled because of the lack of organization and competitive nature lost. oh can't forget the funds. Oh and lol at phil plait for getting owned by joe rogan.

  7. I found this article while doing some research on Cloud Computing!! Wow after i spent 30 mins getting my head round it this one I read your “Ten Mysteries of the Solar System”. I have to say the section about mars made for very interesting reading. Is there any footage of satellites or probes exploding/Failing on there way to Mars.

  8. but aren’t we asking the Hubble to look at things just a little further than 0.3 light years away, like millions of times that??

  9. I ran some numbers that seem to contradict the assumed resolving power of the Hubble. Hubble was used to image Pluto and its moons. You can see Kerberos clearly. Kerberos is believed to be from 13 to 34 km in diameter. I’ll split the difference and use 24 km. Currently we are 7.5 billion km from Pluto. Using your calculation, Hubble detected an object that is 0.001 arc seconds wide.

    OK, still not enough to detect a 0.00002 arc second object, but much better than the assumed 0.1 arc second resolving power listed above.

  10. hubble has imaged charon, pluto’s moon. Its 4.28 billion Km away (minus the distance from earth to hubble), and has a diameter of 1207 km. I calculated that using your equation and got a resolution of 0.05 arc seconds. its not a pretty picture, but it had enough “resolution” to get that. In fact, they used hubble to find a new moonlet of neptune (4.28 billion km away) that has a diameter of 18 km. I calculated that resolution as well, taking into account that its located 1.0525e8 m from neptune. It was 0.001 arc seconds.

  11. Here, I’ll tell you why.. b/c scientists generally don’t seem to understand the difference between the Philosophy of Naturalism and Science.
    The Oort must exist to support a Naturlistic paradigm; that’s regardless that there is no evidence.

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