Planets of Milky Way satellite galaxy stars

[folkhemmet]

Here is a paper that just came out on the astrophysics preprint archive reporting on the results of a search for exoplanets in the Milky Way globular cluster Omega Centuri: http://arxiv.org/abs/0710.3461 Here is the title: THE FREQUENCY OF LARGE RADIUS HOT AND VERY HOT JUPITERS IN ! CENTAURI

I remember reading another paper several years ago--more toward the beginning of this decade--which talked about the chances of planets existing in the Magellenic clouds. The assumption: lower metallicity suppresses planet formation. The conclusion was negative--that is, the fraction of stars with planets in these satellites is probably smaller than the fraction of stars with planets in the galactic disk and bulge. Does anyone remember this paper (sorry for the vague reference)? What are the chances of ETs starring "down" at us from the Magellenic clouds, say?

[Tinaa]

Moved to Life in Space.

[kzb]

Well I was looking at a paper about planets in a globular cluster a few months back. It said that low metallicity may actually increase planet formation, and came up with possibly hundreds of planets per star. Some planets get delocalised within the cluster due to orbital peturbations caused by close stellar approaches.

[kzb]

This was the paper:

http://www.journals.uchicago.edu/ApJ...559/54559.html

[Noclevername]

This was the paper:

http://www.journals.uchicago.edu/ApJ...559/54559.html

Link didn't work for me.


The webpage cannot be found
HTTP 404
Most likely causes:
There might be a typing error in the address.
If you clicked on a link, it may be out of date.

[kzb]

Let's try this:

http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v565n2/54559/54559.html

[Noclevername]

That works, thanks.

[Romanus]

It isn't just the Magellanic Clouds; it seems like every satellite galaxy has a low-metallicity population. That said, I think the chances of exoplanets in said Clouds is far and away better than for some of the other dwarfs like Leo I and the Sculptor Dwarf, to name a few.

[kzb]

The thing is Romanus, we are making an assumption I think, low metallicity= low chance of planets.

Now is this assumption justified? That paper I linked to seems to be saying, no it's NOT justified.

Tau Ceti has significantly lower metallicity than our sun, yet it is reportedly surrounded by an extensive disc of rubble. I don't know however, how the mass of this disc would compare to a system of planets.

I suppose there must be a lower cut-off, it would be unreasonable to expect terrestrial planets if the metallicity was zero for example. However I feel we shouldn't jump to conclusions without evidence to justify those assumptions.

[eburacum45]

I don't get that impression from that paper at all. In fact at one point it says

In the solar neighborhood, the parent stars of planets tend to be considerably richer in metals than average (Laughlin 2000).
and in the Laughlin paper

The existence of a connection between high stellar metallicity and the presence of a short-period planet is one of the most exciting developments that has accompanied the discovery of extrasolar planets.
So, the higher the metalicity, the greater the chance of a short period planet. or so it seems.

One suggestion is that stars with significantly higher metalicities than the Sun will be more likely to have hot Jupiter-type planets, making the existence of terrestrial planets less likely. So we are (as in so may other things) in a goldilocks situation, where too little metalicity would mean a dearth of planets, and too much would mean a hot gas giant instead of terrestrial planets.

The Magellanic Clouds at least have a certain number of supernovae- so I would expect a reasonable number of terrestrial planets there.

[kzb]

Actually eburacum45 I get the opposite impression from the paper. It's true the authors start off by presenting arguments such as the one you quoted, but they are just doing this in order to knock them down later.

This is from the Conclusion:

<<While it should be stressed that the detection of free-floating planets in M22 is preliminary, and also speculative, it suggests that at least 100 planets
were formed for every star. This may sound implausible but is in fact supported by recent simulations. Ida & Kokubo (2001) have shown that in a protoplanetary disk where the surface density of the solid component is low, the isolation mass of planets is small and many terrestrial planets can form. It is also possible that protoplanetary disks, having lower metallicity than solar, would form many earthlike planetsperhaps 50100 per star (S. Ida 2001, private communication). A population of free-floating substellar objects has also been detected in the young cluster Orionis (Zapatero-Osorio et al. 2000). The possibility has been raised that these may be formed as such (Boss 2001), i.e., not attached to a parent star. >>

AND

<<We note that a planet has been detected within a binary pulsar system in M4 (Thorsett et al. 1999) which is metal-poor compared to 47 Tuc (Harris 1996). >>

In fact, I believe the authors are arguing that the non-detection of hot Jupiters is not because they were not formed in the first place, but rather they have been liberated from their natal systems by stellar near-misses in the dense star cluster environment.

I do however note, the paper is largely dependent on computer modelling, and as such it deserves some skeptism. I presented it as I believe there is an unjustified assumption about the number of planets being proportional to metallicity, and the paper is implying this is not necessarily the case.

[eburacum45]

Ah, yes, thanks for pointing that out. The prospect of a system with fifty or more terrestrials is an intriguing one.

That paper is a theoretical discussion of what may happen in globular clusters, but doesn't really argue against the correlation in the galactic disk population. The relationship low metalicity = fewer hot jupiters still seems to apply in the Milky Way, but not in a globular cluster where planets will be perturbed away.

But to address the original post, I don't think that the Magellanic Clouds resemble globular clusters very closely, so the planets there wouldn't get perturbed away so much.

[Noclevername]

...it suggests that at least 100 planets
were formed for every star.
Suddenly the premise of Firefly doesn't sound so far-fetched. :D

[kzb]

I've been trying to look into the metallicity and planet formation issue via Google Scholar. This is an interesting paper, one conclusion being that only 3.7% of planets that exist are actually detectable by current methods !

It also has things to say on detection bias, eg we are much more likely to detect close-orbiting massive planets than other sorts of planets.


http://www.space.unibe.ch/taps/doc/willy.benz.pdf

[Noclevername]

I've been trying to look into the metallicity and planet formation issue via Google Scholar. This is an interesting paper, one conclusion being that only 3.7% of planets that exist are actually detectable by current methods !


But of course, we can't confirm that statement until it's no longer true. ;)

[kzb]

Yes of course that's true Noclevername. You can take issue with the exact value, but when you go through the arguments in the paper, it's clear there must be a large detection bias towards detection of close-orbiting massive planets.

What's more, now that the correlation beween stellar metallicity and the occurence of such planets has been established, you can bet that astonomers in this field are concentrating their efforts on high-metallicity stars, thereby further extending the apparent bias.

However, back to the original question, which was about the likelyhood of ET staring down on us from the Magellanic clouds. If we take that one step back and think about the occurence of terrestrial planets in those galaxies, several papers I have been looking at, including the one I referenced above, make clear that low metallicity is no barrier to possession of terrestrial planets.

In fact the position is the exact reverse: large amounts of metals causes the early and runaway accretion process that leads to inwardly spiralling giant planets, conversely, in low metallicity systems, the planet kernals are more widely separated, so that runaway accretion is less likely. These planet kernals are the precursors of terrestrial planets in such systems.

Hot Jupiters are bad news (on average) for the likelyhood of life-bearing planets. Therefore high metallicity may be bad, and low-to-moderate metallicity good, for the occurence of life-bearing planets. So, going on present information, it's not unlikely there could be Earthlike planets in the Magellanic clouds.