[X] My Assistant

[edstrick]

Middle-term, the really solid determination of the initial mass function for sub-stellar non-planets (BD's and rogue planets) will come from the James Webb Space Telescope. With high sensitivity and resolution in the near, middle and thermal infrared, it will be able to detect low-mass objects in reasonably nearby star forming regions down to very low masses, and get a decent observational (instead of theoretical) plot of luminosity-distribution vs. cluster age to test luminosity/spectral evolution models as a function of mass and age.

*eesh... long sentence warning!*

What we'll really need someday is an infrared version of the under-development Synoptic Survey telescope, to get an infrared sky survey down to something like 30'th magnitude. That would really nail down the "local" population of low mass objects.

[Guest_Richard Trigaux_*]

The point is that, these objects are not only very weak, but also they dwelve in the infrared spectrum, see in radio.

Brown dwarves are in the 1000-2000°C, and as small as Jupiter (Jupiter is about the maximum diametre allowed for a cold body without nuclear reactions)
Large planets could be still colder: Jupiter, without sun warming, could be at something less than 50°K
Small planets, KBO-like, lone comets, fluffy balls, are in thermal equilibrium with the surrounding space. Thus they cannot be detected by their thermal radiation.


The IMF into an interplanetary cloud and around a parent body is likely not the same, as the presence of an accretion disk makes things much faster and at a much smaller scale. So we may expect that the IMF around a star is shifted toward smaller bodies with several orders of magnitude. But observational evidences are still far out of reach...

[ljk4-1]

Large planets could be still colder: Jupiter, without sun warming, could be at something less than 50°K

I thought Jupiter radiated more heat than it receives from the Sun?

[ngunn]

I think that just means it re-radiates what it receives from the Sun plus a little bit more. Richard's estimate would be for the case where that little bit was all it had.

[Guest_Myran_*]

ljk4-1 wrote: I thought Jupiter radiated more heat than it receives from the Sun?

Yes thats correct, i've read that it is thought that the excess energy comes from gravitational contraction. Or in simple terms that Jupiter shrinks a very small bit each year.
(The powerhouse behind winds of Saturn on the other hand might, according to one theory, get the power from helium rain where the latent heat is released when the helium condenses.)

[ngunn]

Since our recent discussion on brown dwarf numbers I've found this on Wikipedia:

"Since 1995, when the first brown dwarf was confirmed, hundreds have been identified. They are now believed to be the most numerous type of body in the Milky Way."

Can anyone help me track this statement to a source, authoritative or otherwise?

[Alan Stern]

Since our recent discussion on brown dwarf numbers I've found this on Wikipedia:

"Since 1995, when the first brown dwarf was confirmed, hundreds have been identified. They are now believed to be the most numerous type of body in the Milky Way."

Can anyone help me track this statement to a source, authoritative or otherwise?

This is a laugh. There are more comets in our Oort Cloud alone (10^13 to 10^14) than there are
brown dwarfs in all the galaxy. If our OC is typical and OCs form around even 1% of the stars, then
there are >10^22 comets in the Milky Way (and that ignores the ones ejected to interestellar space
during OC formation-- probably another factor of 10-30x higher). Planets, comets, and asteroids
out number any and everything stellar/substellar.

-Alan Stern

[Greg Hullender]

Forgive my presumption, but I had thought the Oort cloud was still a hypothesis. How confident are we of those numbers?

[Alan Stern]

Forgive my presumption, but I had thought the Oort cloud was still a hypothesis. How confident are we of those numbers?

Quite confident. Long period comets are observed on OC orbitsall the time: over 800 now. These are the basis of the population estimates, which also agree to an orfder of magnitude with prediction of how much should bein the OC from giant planet clearing.

[The Messenger]

Thanks, Alan.

There is an interesting, but EXTREMELY speculative alternative to the current Oort cloud scenario emerging. Listen at what our probes are telling us:

1) According to Voyager I, the solar wind slows more than expected, and the magnetic field associated with the solar wind increases in strength while decreasing in frequency. Solar wind particles must be piling up more than expected.

2) Both Wild 2 and Temple 1 are populated with fine dust, rich in iron and other minerals not expected to be found in the comets.

It is not difficult to add these observations together, and imagine what amounts to solar raindrops: solar wind particles condense onto Oort and/or Kuiper belt nuclei and rain back into the solar system as comets. Since the 'seed' nuclei is richer in water and other volatiles that condense only in the outer SS, mild heating percolates these volatiles through the dusty crust, and after just a few passes we would get the explosive venting we witness as comets.

For this to be true, the ratios of heavy elements in the solar wind, and what we have observed from comets must be roughly the same.

[Guest_Richard Trigaux_*]

Messenger, I had an intuition of something like that from long ago. I was not convinced by the theory as what Ort cloud objects were in orbit around the Sun. If so, these orbits would be more or less circular, but not so excentric. If all the comets of the Oort cloud where in the very elliptic orbits we see, they would all be burned by the sun for long. If they needed to be disturbed from a more or less circular orbit, They would come in groups.

The idea you propose is much more appealing: the comets are not in orbit, they fall back on the Sun. In the process, they all more or less gather some lateral motion which makes them appear on very elliptic orbit (as a free fall trajectory is none else than a very elliptic orbit).

If the theory of the disturbance was true, only very few comets would have their orbital speed just canceled, and most would be on far orbits. There are too much comets falling right on the sun for this.

[ljk4-1]

Let us hope the Sol system and the rest of the galaxy's stars aren't like
the ones described in a David Brin story (I think it was called "The Great
Silence", but please correct me here).

All star systems are surrounded by a literal shell of comets that needs to
be penetrated by a fast starship in order to be destroyed - usually at the
cost of the ship and any crew and then the system has to endure a hail of
comets for a while.

The story also "explained" why SETI had not been successful until after
the first starship made the first crash through: Only a few other species
had also made the discovery, while others never even bothered to leave
their planets.

[Mongo]

Let us hope the Sol system and the rest of the galaxy's stars aren't like
the ones described in a David Brin story (I think it was called "The Great
Silence", but please correct me here).

All star systems are surrounded by a literal shell of comets that needs to
be penetrated by a fast starship in order to be destroyed - usually at the
cost of the ship and any crew and then the system has to endure a hail of
comets for a while.

The story also "explained" why SETI had not been successful until after
the first starship made the first crash through: Only a few other species
had also made the discovery, while others never even bothered to leave
their planets.

I believe that the story was called "The Crystal Spheres". Another important point in the story was that these spheres could only be broken from within itself, so only those civilizations that sent out a massive, high-velocity (meaning a substantial fraction of c) object -- such as a starship -- would have their enclosing shells broken. Oh, and the shells would absorb all artificial radio-frequency emissions from within themselves.

Bill

[Guest_Richard Trigaux_*]

These stories are interesting, but fictional. It is curious that so many people explain ET behaviours or SETI results with their own behaviour or way of thinking...


The Oort cloud is a threat for spaceships going through it, but no more than our asteroid belt, which was traversed several times by spaceships, without any encounter. But, I remember, not so long ago it was seen as a terrible obstacle forbidding any exploration of planets beyond. I remember seeing a fiction story with asteroids as close together as trees in a forest. I remember it, it was in a story of... Donald Duck.

[Guest_Myran_*]

Richard Trigaux wrote: The Oort cloud is a threat for spaceships going through it, but no more than our asteroid belt.

I would like to voice the idea that the Oort cloud would be far less of a threat.
Even if the number of objects might be quite higher in the Oort
cloud. They are spread out of one far lerger orbit around the Sun, and then not even bound to the ecliptic but spread out in a spherial volume.