Number of Planets in the Solar System

[solidsnake]

How do we know the number isn't 67 or 122?

Is it possible that we can't see these hypothetical distant planets because they are too far?

How firm is Science on the number 8?

 

[wick07]

How do we know the number isn't 67 or 122?

Is it possible that we can't see these hypothetical distant planets because they are too far?

How firm is Science on the number 8?

In an effort to better address your question, could you please define the word "planet".

 

[MeteorWayne]

By the current definition there are 8 planets and 3 dwarf planets. It's highly unlikely there are any other objects that would meet the current criteria to be called a planet in the solar system (not impossible). Any objects that large would most likely have been found in the searches made up until now.
Past Neptune all we have found have been 4 small dwarf planets so far (Pluto, Eris, Haumea, and Makemake). There probably are quite a few other dwarf planets out in trans Neptunian space, at least 50 and maybe more than a hundred. And of course, the dwarf planet Ceres resides in the asteroid belt.

So the number 8 for planets is pretty firm, while the number of dwarf planets will continue to rise for decades.

 

[nimbus]

Am I remembering wrong that there's good odds for a Mars-Earth sized something out in the Kuiper/Oort outskirts?

 

[MeteorWayne]

That was a thought a few years ago, but the most recent evidence indicates the odds are quite low. Not zero, though

 

[matthewota]

Mike Brownhttp://www.mikebrownsplanets.com/ Is still searching for Kuiper Belt objects. It is an ongoing science.

 

[3488]

That was a thought a few years ago, but the most recent evidence indicates the odds are quite low. Not zero, though

Yes that's quite true.

The more observations are increasingly making the existance of a Moon, Mercury, Mars, Venus or Earth sized KBO looking less likely.

Because accretional rates are so slow that far out from the Sun it is looking like that Eris is indeed towards the upper limit of the size / mass range for individual KBOs.

Eris indeed may not be the actual largest or most massive body within the KB, but may well be very close to that point.

Andrew Brown.

 

[MeteorWayne]

The wild card here is that most of the objects in the Oort cloud are suspected to have formed in the inner solar system among the Gas Giants, and were ejected to the Oort cloud through gravitational interactions with the planets. So there's always a chance, but we are examining this area more thoroughly now. When PANSTAARS comes online, there will be a very deep (dim) census taken of that part of the solar system, so the odds should decrease rapidly.

 

[MetalMario]

Well, since the discovery of the current planets, we haven't found any. Possibly there are planet that orbits at the same speed as us, behind a planet or the sun.

There are so many Dwarf Planets being discovered. I don't think it will be long before we find a Mars-Sized object.

 

[MeteorWayne]

It wuld be impossible for a planet to hide behind another one. Due to simple orbital mechanics, every planet orbits at a different speed depending on it's distance from the sun (Mercury 88 days, earth 1 year, Jupiter 12 years, etc.)

A planet hiding behind the sun would have to have the exact same eccentricity as the earth's orbit or it would have peek from behind, very unlikely. In addition, we have had spacecraft all over the solar system and it would have been spotted.

 

[CalliArcale]

It wuld be impossible for a planet to hide behind another one. Due to simple orbital mechanics, every planet orbits at a different speed depending on it's distance from the sun (Mercury 88 days, earth 1 year, Jupiter 12 years, etc.)

A planet hiding behind the sun would have to have the exact same eccentricity as the earth's orbit or it would have peek from behind, very unlikely. In addition, we have had spacecraft all over the solar system and it would have been spotted.

The idea of a planet hiding in the Sun-Earth L3 point is an old one, of course. It was popular for science fiction in the early 20th Century, but was never seriously entertained by astronomers. Even before we had spacecraft capable of peeking at the L3 point, we had good enough charts of the motions of other bodies in the solar system. If anything lies at the L3 point, it would have to be very small because it clearly has no significant gravitational affect on anything else.

However, there has never been a mission *specifically* to look at L3. Many missions have had the opportunity, though they've been aimed at other things. A spacecraft light SOHO, positioned at L1, would have no better luck than we would, but the Ulysses spacecraft, orbiting the Sun in a highly inclined orbit at roughly the same distance as Jupiter, would have been able to see our L3 point easily. However, it was not an imaging spacecraft. The twin STEREO (A and B, for "ahead" and "behind" respectively, which orbit the Sun in orbits similar to the Earth's) can see L3. There has been no report of a planet discovery. And then of course there are all the planetary probes. Mars Global Surveyor took images specifically of Earth from Mars. Cassini (orbiting Saturn) did the same. Voyager 1 made the coolest image of Earth, as part of its carefully-planned solar system portrait taken on Valentine's Day, 1991.

But as far as I know, no spacecraft has made a detailed survey of the Sun-Earth L3 point. It is possible, though pretty unlikely, that there's something hiding there. It would have to be something very small, though, and not a major planet.