Comments on: What Makes A Planet A Planet?

By: Brian Koberlein

Brian Koberlein — Tue, 09 Aug 2016 13:00:44 +0000

Under your definition wouldn’t be a planet, since everything technically orbits not the Sun but the barycenter of system. For the IAU, they mean orbits the Sun in the traditional sense, including Jupiter. Jupiter is considered a planet, not a star, because it isn’t massive enough to generate nuclear fusion in its core.

By: dell inspiron

dell inspiron — Tue, 09 Aug 2016 06:39:40 +0000

base on the IAU definition of planet,
1. is orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has “cleared the neighborhood” around its orbit.

Doesn’t that make Jupiter not a planet too? It clearly doesn’t meet criteria #1
it so massive, it doesn’t obit the Sun but it orbit a point in-space together with the sun.

By: Valbona Pupa

Valbona Pupa — Wed, 17 Feb 2016 00:17:58 +0000

Hi Professor.On your post,you mention (esteroid belt).Yes.The entry of Milky Way have huge and thick esteroid spin around side by side and to the center a huge Planet.I have more to tell you,and I hope you can get this.

By: genialityofevil

genialityofevil — Tue, 16 Feb 2016 22:11:45 +0000

You can have binary planet systems but such a broad definition makes our system too complicated because the range of properties of what would be called ‘planets’ would just be too great. The whole point of scientific definitions is to make the delineation between objects clear.

By: Jpatrick

Jpatrick — Tue, 16 Feb 2016 21:28:19 +0000

Yes. Who says a planet can’t be a moon?

By: genialityofevil

genialityofevil — Tue, 16 Feb 2016 19:38:20 +0000

Which would make the Moon a planet, along with most of the other moons, including Charon. So now most of the planets are actually multi-planet systems and Jupiter starts looking like a mother hen.

By: Jpatrick

Jpatrick — Tue, 16 Feb 2016 19:20:32 +0000

The guidelines for defining a planet were drawn up as if there were no known exoplanets, although as of 2006, I think there were. More precisely, there were suspected exoplanets, but perhaps unconfirmed.

Since there are now thousands of known exoplanets, the guidelines have to be reviewed.

I’d be inclined to just adopt the single criterion of being in “hydrostatic equilibrium”, with some constraint on how spherical the object is. As long as the object’s mass is low enough not to begin fusion, it should be regarded as a planet. In this view, a planet doesn’t need a star, and it doesn’t need a cleared orbit.

By: Raul

Raul — Tue, 16 Feb 2016 14:28:51 +0000

What about ROGUE (solitary) planets ? And about 3rd criterion, is it ACCRETION the unique process of formation of a planet (despite simulations)? TYSM

By: genialityofevil

genialityofevil — Tue, 16 Feb 2016 13:36:38 +0000

Is there a reason why inclination is not considered part of the definition? The regular planets are all within a few degrees of each other but Pluto (and Eris) are much further off the plane.

By: Wayne

Wayne — Mon, 15 Feb 2016 15:14:14 +0000

Recently saw an article on Kepler finding a rocky planet half the diameter of Neptune – wonder how unusual this will be as we continue to detect more and more exoplanets: http://www.universetoday.com/127325/largest-rocky-world-found/

By: Elver S.S.

Elver S.S. — Mon, 15 Feb 2016 14:23:46 +0000

One correction to my question :)…it should read unnecessary.

.elver

By: Elver S.S.

Elver S.S. — Mon, 15 Feb 2016 14:22:43 +0000

Dr. Koberlein, one potential correction and one question:

Potential correction: This sentence…
Things get interesting if you drop the IAU’s _second_ criterion of needing to orbit the star

should read

Things get interesting if you drop the IAU’s _first_ criterion of needing to orbit the star

Question:
it seems that criteria 3 (clearing the neighborhood) makes criteria 2 (hydrostatic equilibrium) necessary. Is this a correct assumption?

.elver