[meteorite-list] It's a star, it's a planet, it's a 'planemo'

From: Larry Lebofsky <lebofsky_at_meteoritecentral.com>
Date: Wed Jun 7 11:19:29 2006
Message-ID: <1149677687.4486b0773cc74_at_hindmost.LPL.Arizona.EDU>

Hi again Sterling:

While I normally do not disagree with you (never did get back to you on Planet
V, but that is another issue), I think the issue here is a matter of physics
and a matter of nomenclature.

While the issue of what is a planet (orbiting a star is not as clear cut), it
is less so for things "out in space."

By the way, I know a number of the people on IAU and they are not all French.
The commission that deals with planets and satellites is actually dominated by
people from the US with mamembers from a number of countries including the
Vatican.

If you are big enough to burn hydrogen, you are a star

If you are big enough to burn deuturium, you are a brown dwarf.

If you are smaller than that, you are a sub-brown dwarf (as far as I know
includes things that are "planet-sized, whatever that means. It is unlikely
that we will ever observe things as small as Earth or Pluto, at least in the
near future). This has already been an issue when someone "discovered" a free-
floating "planet" that turned out just to be a brown dewarf.

I agree with your statement that any object that is not in orbit around
another object (star) is not a planet. However, I think you went too far with
all of your decimal points. I am not a stellar physicist and as far as I can
tell, the brown dwarf/planet size boundary keeps changing as models get more
detailed.

With respect to your examples, when do call two thing revolving around one
another a binary system and when do you call such a thing a planet and a
satellite or an asteroid and a satellite? This I cannot answer.

Nature does not define planet/asteroid/comet, etc. At some level, though, it
(and physics define star). So, at some pint if you have to deal with names and
claims, you have to come up with nomenclature: it is a fact of life. You want
it to be physically meaningful, as in the case of planet vs. asteroid. In the
case of free-floaters, it avoids someone coming along and "naming" planets
that have escaped from stars or from coming up with a "new class of objects"
the planemos (which is what started this) or asteros or cometos (or is that
comatose). If this naming thing is a game, talk to the biologists, gthey do
much more of this.

Maybe we can recycle the term asteroid since the "proper" term is minor
planet, which really does make sense (minor star?).

Larry

Quoting "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>:

> Hi, Larry, List
>
>
> I never argue with old Isaac.
>
> Well, the IAU has its agenda. Being entirely French,
> their agenda is to equivocate as long as possible and then
> just a bit longer to be absolutely sure to avoid any
> embarrassment...
>
> The problem is that the definition of "planet" is both
> endogenous and exogenous, by innate characteristics
> (size, composition, temperature, etc.) and circumstantial,
> orbitally subservient to what other object. I suppose that
> any object that is not in orbit around another object that
> is not a star is not a planet.
>
> So, according to the IAU, an 8-Jupiter-mass body
> in orbit around a 15-Jupiter-mass body is a planet, since
> the 15-Jupiter-mass body can fuse deuterium and the
> 8-Jupiter-mass body cannot. But if the 8-Jupiter-mass
> body is on its own, it's a star.
>
> Have I got that right?
>
> What if it's a 7.99-Jupiter-mass body in orbit around an
> 8.00-Jupiter-mass body? Is one a planet and one a star,
> both cold, dark, and dead? Or is it a sub-brown-dwarf
> binary system?
>
> What if it's a 1-Jupiter-mass body on its own? Is it
> still a star, a sub-sub-sub-brown-dwarf, a cold, dark,
> dead star? It's not a planet...
>
> Hey, maybe it's an asteroid! Since deuterium burning
> is only possible at 12 or 13 Jupiter-masses, I guess an
> 11.99-Jupiter-mass body is like, the Universe's biggest
> asteroid!
>
> See, we went and wasted "asteroid" on minor planets,
> when it literally means "tiny star," ASTER being Greek
> for "star." It would have been the perfect terminology!
>
> This definition game is tiring, like playing handball.
> My wrists hurt. The IAU can have it.
>
>
> Sterling K. Webb
> -------------------------------------------------------
> ----- Original Message -----
> From: "Larry Lebofsky" <lebofsky_at_lpl.arizona.edu>
> To: "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>
> Cc: "Meteorite List" <meteorite-list_at_meteoritecentral.com>
> Sent: Tuesday, June 06, 2006 12:38 PM
> Subject: Re: [meteorite-list] It's a star, it's a planet, it's a 'planemo'
>
>
> > Hi Sterling:
> >
> > 1. According to the IAU, there are no free floating planets. Their
> > official
> > name is "sub-brown dwarf." This is probably to avoid people trying to name
>
> > them
> > or run into problems when you really do not know their mass acurately and
> > so
> > they may just be on the smallish end of brown dwarves.
> >
> > 2. What is the difference between an object orbiting another and the two
> > revolving around each other? Thanks to Newton, any two objects revolve
> > around
> > their center of mass. So, for example, the center of mass of the
> > Jupiter/Sun
> > system is 46,000 km OUTSIDE the surface of the Sun. So does Jupiter orbit
> > the
> > Sun or do they revolve around one another?
> >
> > Larry
> >
> >
> > Quoting "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>:
> >
> >> Hi,
> >>
> >>
> >> It's a star, it's a planet, it's a 'planemo'
> >> http://news.com.com/Its+a+star,+its+a+planet,+its+a+planemo/2100-11397_3-
> > 6080197.html
> >>
> >> Too lightweight to be stars but bigger than most planets, a handful of
> >> hot,
> >> young, free-floating objects have the raw materials to make their own
> >> miniplanetary systems, astronomers reported on Monday.
> >>
> >> Just like some young stars, these so-called planemos have discs of cosmic
> >> dust and gas circling them. These kinds of discs contain the ingredients
> >> for
> >>
> >> planets; astronomers believe Earth and the other planets in our solar
> >> system
> >>
> >> were forged from such a disc.
> >>
> >> But planemos--short for planetary mass objects--are unlike normal planets
> >> because they do not orbit stars, said Ray Jayawardhana of the University
> >> of
> >> Toronto. He and other researchers presented their findings at a meeting
> >> of
> >> the American Astronomical Society in Calgary, Alberta.
> >> "These things are not orbiting a star. They're by themselves,"
> >> Jayawardhana
> >> said in a telephone interview.
> >>
> >> The researchers detected four newborn planemos, just a few million years
> >> old, in a star-forming region about 450 light-years from Earth, a
> >> relative
> >> stone's throw in cosmic terms. A light-year is about 6 trillion miles,
> >> the
> >> distance light travels in a year.
> >>
> >> All four of these objects had dust discs around them, the astronomers
> >> reported.
> >> Scientists also found a disc-skirted planemo interacting with a brown
> >> dwarf--a failed star--even closer to Earth, just 170 light-years away.
> >>
> >> Such a planet-sized object might have been expected to be pulled into
> >> orbit
> >> around the brown dwarf, but instead the two revolve around each other,
> >> and
> >> both have the makings for more satellites.
> >>
> >> These objects, with several times the mass of the giant planet Jupiter
> >> but
> >> 100 times less massive than our sun, are cosmic infants only a few
> >> million
> >> years old.
> >>
> >> Even Jupiter had a disc when it was young, and its dozens of moons were
> >> formed from the dust and gas it contained. However, Earth's rocky moon
> >> probably was born when our world collided with another heavenly body
> >> early
> >> on, and Mars' moons were asteroids captured by the planet's gravity.
> >>
> >> But planemos are a relatively new player on the cosmic scene, filling the
> >> gap between the least massive stars and the most massive planets,
> >> Jayawardhana said.
> >>
> >> "These are the lowest-mass brown dwarfs or really big giant planets,
> >> especially when they're young," he said.
> >>
> >> When young, planemos are still warmed by the heat of formation and are
> >> more
> >> like stars, he said. But as they age, these planet-esque objects shrink
> >> and
> >> cool.
> >>
> >> Other researchers do not use the term "planet" to describe any satellites
> >> that might be formed around a planemo, referring to these as moons or
> >> moonlets.
> >>
> >> If such bodies do form, they would be inhospitable to Earth-type life. If
>
> >> a
> >> satellite formed very close to a young planemo, it might be temporarily
> >> warm
> >>
> >> enough for liquid water to exist, and water is a requirement for earthly
> >> life.
> >>
> >> But Jayawardhana acknowledged that in the long run, life would have dim
> >> prospects: "Any kind of planet that forms around them is committed to an
> >> eternal freeze."
> >>
> >> Story Copyright ? 2006 Reuters Limited. All rights reserved.
> >>
>
>

-- 
Dr. Larry A. Lebofsky
Senior Research Scientist
Co-editor, Meteorite                      "If you give a man a fish,   
Lunar and Planetary Laboratory               you feed him for a day.
1541 East University                       If you teach a man to fish,
University of Arizona                        you feed him for a lifetime."
Tucson, AZ 85721-0063                                     ~Chinese Proverb
Phone:  520-621-6947
FAX:    520-621-8364
e-mail: lebofsky_at_lpl.arizona.edu

Received on Wed 07 Jun 2006 06:54:47 AM PDT