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Comment by vivekd | Hacker News Reader

It's interesting how space research technology works, right now we can detect planets around distant stars many light years away but we are still unable to detect a planet several times larger than earth that is likely orbiting our own sun.

I wonder if it's possible to send a probe out beyond the orbit of Neptune and try and detect planet 9 using gravitational lensing the way we do with planets in other solar systems.

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When surrounded by pitch dark the human eye is capable of spotting the light of a candle from tens, possibly hundreds miles away at a clear night, and yet under those conditions it would also be perfectly possible to hide an elephant a few meters away from someone

Why can’t we look for moving dim spots in the cosmic microwave background?

The CMB consists of longer wavelengths than light (around 1 mm compared to 500 nm, or 2000 times larger), so a radio telescope capable of resolving an angle as small as a small planet 100 AU away would be enormous, kilometers in diameter.

It's also a lot easier to point a telescope at the extremely specific points. Even if the CMB could be used as effectively as stars, you can't aim a directional telescope at the entire sky all at once. You have to pick a spot to stare at & get lucky.

Try looking out the back of your head for the planet hiding right behind you. Once you figure out that you have to turn around, the planet might not be there anymore :) We could build telescopes & point them in every direction though!

The issue seems to be one of illumination... as unlikely as it is, would it help to off a big bright bomb or two as "flashlights" to speed up the search?

It's already illuminated by the huge fusion bomb at the centre of the Solar System. We can't make bigger or brighter bombs than that.

Sure you can. Its inverse-square law which extinguishes the brightest flame (except of course a nova or supernova). A much nearer explosion would be much, much brighter. But you have to have some idea where to set it off.

So if we knew the answer to our question (where to set it off) it would be easy to get the answer to the question (where that object is).

Well, yes. True. Not spectacularly informative though.

I call bullshit. The horizon is only about 3 miles away.

Elevation significantly increases visibility, such that the longest photographed distance on Earth is 443km :)

You make a valid point (although another counter-argument has already been given.

But speaking purely in terms of human eye sensitivity this has been confirmed in an indirect way at least: the faintest star visible to the human eye (under perfect conditions) is comparable in light intensity.

EDIT: also, sky lanterns https://en.wikipedia.org/wiki/Sky_lantern

> It's interesting how space research technology works, right now we can detect planets around distant stars many light years away but we are still unable to detect a planet several times larger than earth that is likely orbiting our own sun.

The planets we're detecting are all large, and relatively close to their stars - so much so that we can observe the star's brightness changing, as the planet passes in front of the star, or that we can observe the gravitational wobbles of the star, as the planet orbits it.

We would not be able to detect a Planet 9 orbiting around, say, Alpha Centauri.

Even planets like Jupiter and Saturn are unlikely to have been found around other stars yet (with the transit method) because their orbital periods are 12 and 30 years respectively. We simply haven't been looking long enough to see multiple passes.

When researchers look for a planet around a star, they only need to look at a small part of our sky. Not so for nearby objects.

Plus, they can exploit the fact that stars with planets nearby will exhibit patterns of brightness, etc.,—if the planet is in the right orbit, has a great deal of mass, along with other parameters.

Also they're not usually looking for a specific planet around a particular star. The sky is full of stars and planets to find but there's only one planet nine.

Why do you assume that there is only one planet nine?

Are you using the IAU definition of a planet? If so, why assume that there even is one? And if you are using a different definition, why assume that only one single object meets those criteria?

If you are willing to forego "cleared the neighbourhood" then there might be dozens of round worlds with upwards of 10^20 KG mass in the solar system. We even know of such worlds not bound to any star at all!

Even if there are multiple undiscovered planets, there would be at most one planet nine, plus maybe a planet ten, eleven, etc.

What would happen if we discover an object that deserves the term "planet" at 180 AU, and then five years later discover another at 120 AU? Which one is "nine"?

Both. The one at 180 AU would be considered the 9th known planet from the Sun, until the one at 120 AU was discovered.

It'd have to be a really heavy planet to see obvious gravitational lensing effects (I.e. Sun size).

We detect planets in other systems by seeing them pass in front of the host star, or we detect them radiating IR, or if exoplanet is really big, with small orbit, it's gravitational influence on the host star (I.e. see it 'wobble').

In all cases, exoplanet is quite close in orbit to star.

By contrast, if a big planet is orbiting sun, but really far away (>x100 earth-to-sun-distance), it'd be to cold to detect it radiating any IR, it'd be too far to see obvious effects of it's gravity.

Does that technique find Kepler Elements of small, remote planets? Or just demonstrate the existence of large planets?

In some cases, we're detecting the wobble in the star's light caused by the orbiting planet's gravity. We're not directly imaging the planet itself.

Gravitational lensing? Planet-sized objects arent massive enought for that to be useful.

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