“The great oak of Astronomy has been felled, and we are lost without its shadow.” –Subrahmanyan Chandrasekhar, on the passing of Jan Oort
In our new semi-regular series, readers from all over the world (and in low-Earth orbit, too, because why not!) are invited to send in their questions and suggestions for a chance to have them answered here on this blog! Today, our question comes from Robert Meegan, who asks,
Have any Oort Cloud objects been detected in situ, as opposed to when they whipped through the inner solar system as comets? I would presume that any detection would need to be indirect, through occlusion or microlensing. Thanks for the great blog.
This is a good one! You might know that it’s more than four light years to the nearest star, but our Solar System, the way we typically think of it, only goes out for about 50 times the Earth-Sun distance (known as an astronomical unit, or A.U.), which is just 0.08% of a light year!
The objects that lie out at this distance — in the Kuiper Belt — occasionally get gravitationally perturbed, often by Neptune, and slingshotted into the inner Solar System, where they can become comets if they get too close to the Sun. But this is not the only place that comets come from!
Beyond the Kuiper Belt, a long way beyond the Kuiper Belt, lies the Oort Cloud, some 50,000 A.U. — or nearly a light year — away from our Sun. Every once in a while, we get a comet that comes through our Solar System, and instead of having a period of just under a century, more or less, which is what you’d expect if a comet came from the Kuiper Belt, it has a period of more like 100,000 years! These long-period comets, like Comet McNaught (above), are large in number, and are unlikely to have their orbits result from an encounter with Neptune.
Instead their origin is from a cloud of objects a long, long distance away! The Oort cloud was hypothesized by Jan Oort in 1950, and is generally accepted to be the point-of-origin of these long-period comets. Because of the incredible distance to the Oort cloud, and the fact that these objects have neither any intrinsic luminosity (even in the infrared) nor are they close enough to reflect a substantial amount of sunlight, they have remained invisible at their great distances.
But is there any way to detect them? In principle, yes.
Gravitational Microlensing is when a non-luminous object passes in front of a bright star, temporarily magnifying the starlight as it transits across the surface of the star as seen by our perspective. Unfortunately, the masses of most Oort cloud objects are so tiny — on the order of 1015 kg, or about one billionth the mass of the Earth — that the amount of microlensing that they’d cause is too small by a factor of about a thousand with current technology.
So no Oort cloud object has ever been detected in the Oort cloud; it’s only when they come close enough to be seen, optically, with a telescope that we’ve been able to detect them.
But not every Oort cloud object that has been detected has become a comet, either. There’s one very, very important exception: the minor planet Sedna, as imaged by the Hubble Space Telescope above!
Unlike most Oort cloud objects (based on the comets we’ve seen), Sedna is huge, about 1,000 km in diameter and estimated to be about 1021 kg, or a million times a typical comet’s mass. And the only reason we were able to detect it when we did was because they were looking for objects out beyond Neptune; they just happened, in 2003, to find one farther out than all the other objects!
Sedna only speeds through its orbit once every 11,000 years, indicating an origin far beyond our inner Solar System, and yet it’s gravitationally bound to us! The key — since it never reaches the Kuiper Belt — is that it never could have interacted with Neptune, indicating an origin in the Oort cloud, the only non-comet ever to be detected from out that far!
I hope that this also answers Joe Stevano’s question:
If Sedna is not the most fascinating known body in the solar system, I don’t know what is. I look forward to you writing about it one day.
We can argue whether Sedna is “in” the Solar System or not, but I let gravity be the judge: if it lets you in, I’ll let you in!
And so even though we’ve never detected an Oort cloud object in the Oort cloud directly, we know of one that came from there, that’s headed back, that’s never been a comet. And that’s today’s Ask Ethan!