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The new definition of a planet - as agreed at a meeting of the International Astronomical Union in Prague in August 2006 - is...
"A celestial body in orbit around the sun that has sufficient gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (almost round) shape and has cleared the neighbourhood around its orbit."
Accordingly, there are now only eight planets in our solar system and Pluto has been demoted to being a �dwarf planet' along with Ceres and 2003 UB313 (nicknamed Xena).
Now that Pluto has been dismissed from the list of planets, here's a mnemonic for remembering the ones that remain in their order out from the sun..."Mary's Violet Eyes Make John Stay Up Nights"...Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
"A celestial body in orbit around the sun that has sufficient gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (almost round) shape and has cleared the neighbourhood around its orbit."
Accordingly, there are now only eight planets in our solar system and Pluto has been demoted to being a �dwarf planet' along with Ceres and 2003 UB313 (nicknamed Xena).
Now that Pluto has been dismissed from the list of planets, here's a mnemonic for remembering the ones that remain in their order out from the sun..."Mary's Violet Eyes Make John Stay Up Nights"...Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
I'm no cosmologist, but surely, if there were other celestial bodies in our galaxy big enough to fit the definition I offered in my earlier response, we would know about them. After all, astronomers have established the existence of planets in other galaxies altogether.
Pluto was 'downgraded' on the basis of its size...ie not being big enough...and we've known about it for some time. I'd imagine any others out there would be even smaller, thus qualifying for only 'dwarf' status.
Pluto was 'downgraded' on the basis of its size...ie not being big enough...and we've known about it for some time. I'd imagine any others out there would be even smaller, thus qualifying for only 'dwarf' status.
Detecting more distant planets in our solar system is not as easy as you think. Most extra solar planets are detected via their gravitational influence on the star they orbit. Now we can't do this for our solar system because there are already 8 known planets and trying to extrapolate the existence of any others by accounting for the combined gravitational effect of what we know about already is waaaaaay beyond our mathematical capabilities (in fact just having a 3-body system makes the maths involved pretty much impossible).
Another method is if we see a planet pass in front of a star's disk, astronomers can measure the change in the apparent magnitude of the star. This method obviously can't be used for our system.
Then there's good old direct imaging. For extra solar planets, this method can be used if the planet is of jupiter-size or larger and hot (so it emits a lot of infra-red radiation). Small, low-albedo planets cannot be detected this way. Pluto is faint enough as it is, and the solar system is way bigger, and I mean waaay bigger than Pluto's orbit. Pluto orbits at an average of 40AU, and the minumum boundary of the solar system is 130AU (where the solar wind extends to), and it's upper boundary is anything up to 100,000AU at the Oort cloud. There could be plenty of planet-sized objects out there with a low albedos that make it (currently) impossible to image.
There are other methods, such as spectroscopy and microlensing but these too are unsuitable for our neighbourhood.
Another method is if we see a planet pass in front of a star's disk, astronomers can measure the change in the apparent magnitude of the star. This method obviously can't be used for our system.
Then there's good old direct imaging. For extra solar planets, this method can be used if the planet is of jupiter-size or larger and hot (so it emits a lot of infra-red radiation). Small, low-albedo planets cannot be detected this way. Pluto is faint enough as it is, and the solar system is way bigger, and I mean waaay bigger than Pluto's orbit. Pluto orbits at an average of 40AU, and the minumum boundary of the solar system is 130AU (where the solar wind extends to), and it's upper boundary is anything up to 100,000AU at the Oort cloud. There could be plenty of planet-sized objects out there with a low albedos that make it (currently) impossible to image.
There are other methods, such as spectroscopy and microlensing but these too are unsuitable for our neighbourhood.
I have no dobt whatever but that you are correct, Ryepie, but the questioner specifically asked about planets in our solar system (quote) "these days". The answer to that is "8" in exactly the same way as the answer to the question "What lies to the west of the Atlantic Ocean?" used to be "The East Indies."
Of course, in future days, we may have to revise the figure of 8 to whatever developing science tells us, just as Columbus's voyage forced 15th century people to revise their ideas.
Questions of this nature inevitably involve an element of 'as far as we know'.
Of course, in future days, we may have to revise the figure of 8 to whatever developing science tells us, just as Columbus's voyage forced 15th century people to revise their ideas.
Questions of this nature inevitably involve an element of 'as far as we know'.
ryepie, The existence of both Neptune_and_Pluto remarkably, were virtually 'discovered' through mathematical analysis long before computers of perturbations (irregularities) in the orbits of Uranus and Neptune, respectively.
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