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X CANARY ISLANDS WINTER SCHOOL OF ASTROPHYSICS "Globular Clusters"

PLANETS IN GLOBULAR CLUSTERS

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What would the sky look like if observed from a planet of a globular cluster star? Would this be feasible?

R.ELSON:

"Densities in the cores of globular clusters range from 103-104 solar masses per pc3. Thus, while we have a few stars within 1 pc of the Sun, in the centre of a globular cluster, there would be thousands. The sky would be spectacular indeed, but there would be a high price to pay. Sigurdsson (1992, ApJL 399, L95) discusses ways of forming planetary systems in globular clusters, as well as the prospects for them surviving disruption by close encounters with other stars. He concludes that in the core of an average cluster like 47 Tuc, a planet at 1 AU would survive for only a few hundred million years – an order of magnitude less than the time it took us to involve.

V. CASTELLANI:

"Let me hope to be in a planet at the very periphery of a cluster, out in the galactic halo. If the axis of the ecliptic is not directed toward the clusters, or few months we will have a rather normal, though rather desert sky, possibly with the view of the whole Galaxy over our heads. However, for the restant time the sky would be dominated by the cluster. At a distance of 100 pc from the cluster centre a giant stars in the centre will appear of about mV=2. One can estimate that an old cluster of 105 stars have an absolute V magnitude of the order of mV=-9. Assuming this light coming from the central region one finds a total magnitude mV=-4., that is about 10-8 of the flux we receive from the Sun, much fainter than the moonlight which reaches mV=-13. Thus one concludes that at such a distance the cluster will appear in the night sky quite impressive, but without dramatic consequences. However, in the centre of a dense cluster the stellar density can be as high as 104 stars for cubic parsec. If a planet can survive the close encounters with the surrounding stars, then one expects in the sky several thousands of stars even much brighter than mV=1, against 12 in our sky. Note that we will have, in mean, about 10 stars nearer tan 0.1 pc- Even if dealing., as most probable, with solar type MS stars, they will each be brighter than mV=-5: really a hell of stars. It would be quite interesting to investigate the mythology of people on similar planets!"

M. FEAST:

"I think this depends on which cluster you chose to live in and whereabouts in the cluster. The centres of some clusters are pretty crowded with stars. Even away form the cluster centre there would probably be some pretty bright stars in the sky but one might live confortably round a sun there and it would certainly be a wonderful place from which to do research on globular clusters! One might however conclude that all globulars were like the one that one was living in, and that would, we know, be quite wrong."

W. E. HARRIS:

"Could we expect to find Earth-type planets (big things with rocky/metallic compositions) around globular cluster stars?

In very low-metallicity clusters, probably not; we’d end up with Moon-sized planets at best, after boiling away all the gas; and these little things would not hold on to their atmospheres long enough for life to develop. But in high-metallicity clusters such as many of the ones in the Galactic bulge, big terrestrial planets should be able to form. However, then we need to ask where in the cluster we should look! The central parsec or so, with its frequent star-star interactions, would be a very dangerous environment for planets, which would be removed by tidal encounters. So we’ll have to stay a few parsecs out form the cluster centre, and hope that our star doesn’t have a plunging orbit that would take it through the core every few million years.

Then what would the night sky look like from our hypothetical planet? The core of the cluster would look like a huge nest of multicolored jewels, several degrees across on the sky and almost as bright in total as the full Moon. Inside the core, the main-sequence turnoff stars would be easily visible to the eye as 4th to 5th magnitude – thousands of them sitting on top of the diffuse light of the still fainter stars. But the real spectable would belong to the hundreds of horizontal-branch stars, each as bright as Spica or Altair; and best of all the additional hundreds of yellow and red giants, each shining as brightly as Venus of Jupiter. We should even be able to see the core in the daytime! Scattered more thinly across the sky – but still adding up to thousands of stars visible to the eye – would be the rest of the cluster. And, let’s not forget the Galactic bulge! We are closer to it now, and it is less obscured by dust clouds in the disk, so it would be hanging (somewhere!) in the night sky, again about as bright as the full Moon but much more diffuse. The rest of the Milky Way would stretch across the sky brighter than we see it from Earth.

All in all, a dramatic place to live! The astronomers there must be a happy crowd. And of course, it may be even more fun for us to speculate about how this view of the sky would affect the mythology, religion, and cultural history of any civilisation there.

Isaac Asimov gave a vivid – an mostly correct – answer to this question about half a century ago in one of his fiction pieces called "Nightfall", probably the most famous science-fiction short story ever written."

I. KING:

"Countless bright stars. But of course you couldn’t go there."

R. GRATTON:

"Roughly, stellar density in a globular cluster core is 105 stars per pc3. About 2% of these stars are giants. On average the closest star is at 0.02 pc (4000 astronomic unit), and the closest giant (the brightest star) at 0.08 pc. The brightest star (but the local "Sun") have typically an apparent magnitude of mV=-10 (or like), and there would be a few thousands of stars brighter than mV=-5. Sky would be very bright, but no more than at full moon. Extended planetary systems are unlikely to exist in the cluster cores, but since a significant fraction of primordial binaries is present in the outer regions, also planetary systems may be present there (albeit the effect of the low metallicity on planet formation is unclear)."

S. MAJEWSKI:

"It would look very different indeed! Of course, it would depend a bit on where in the cluster the home star would be when you looked, but in a typical place you would see a sky dominated by hundreds of thousands of yellow to red giant stars, with a few, less bright blue stars (some variable as RR Lyrae type stars) sprinkled in. In a big cluster, you could have a thousand of these stars brighter than apparent magnitude -5 (brighter than Venus) and with the closest as bright as apparent magnitude -10!. Interestingly, the most nearby giant stars would be some 10 arcseconds across — resolvable with a small telescope or binoculars. As you move to the outskirts of the cluster, the distribution of stars would concentrate to one direction of the sky. The 47 Tucans would probably conclude that the universe was 10 parsecs across!

Still, it is not clear whether planets could form in low metallicity systems typical of the Galactic globular cluster system. Even gaseous, Jovian planets are thought to condense around rocky cores. Maybe some of the disk clusters like 47 Tuc might be enriched sufficiently for planet formation."

R. CANAL:

"If we were to find ourselves on a planet going round a star inside a globular cluster, how the sky looked would depend on how near we were to the centre of the cluster. Close to the centre, the sky would appear to us to be completely covered in stars, in all directions; at the edge of the cluster, however, depending on the rotational axis of the planet, either an entire hemisphere or a wide stretch of sky, depending on the season of the year would reveal an enormous density of of stars. It looks as though the regions of greatest star density, in the centre of the globular clusters, conditions are not favourable for the formation of planets: tidal forces due to nearby stars would tend to pull apart any possible protoplanetary discs. However, given the detection of planets surrounding such hostile objects as pulsars, I wouldn’t care to discard their existence out of hand."

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