Globular clusters held an early fascination for me, and I guess anyone who encounters these rich cities of stars for the first time wonders what it would be like to be on a planet deep inside one of them. The clusters appear to be distributed in a spherical halo around the galactic center, ancient collections of stars much lower in heavy elements than stars in the galactic disk (although globular clusters in some other Local Group galaxies seem younger). The thought of the night sky on a planet embedded in such a place makes the mind reel, star upon star upon star filling the view.
Image: The globular cluster 47 Tucanae, the second brightest globular cluster orbiting the Milky Way (behind Omega Centauri). Imagine the night sky deep within such a cluster. Credit: South African Astronomical Observatory.
But a new paper suggests that at least one category of planets may be rare in such clusters. It follows up on an earlier survey of the cluster 47 Tucanae which examined some 34,000 stars and came up empty. The theory here is that the high density of stars in globular clusters disrupts planetary orbits. Even more significant, tidal effects upon planets much closer to their star than Mercury is to our Sun eventually cause ‘hot Jupiters’ to experience orbital decay and an early demise. Add this to the low metallicity (few elements heavier than hydrogen and helium) in globular clusters and you have an environment not conducive to planet building in the first place.
Brian Jackson (NASA GSFC) puts it this way: “Globular clusters turn out to be rough neighborhoods for planets, because there are lots of stars around to beat up on them and not much for them to eat.” Working with colleague John Debes, Jackson argues that any ‘hot Jupiters’ in globular clusters would be destroyed quickly by tidal effects if nothing else, their orbits gradually moving closer to their star until the planet is torn apart by the star’s gravity or crashes into it.
Thus we can explain the dearth of planets in 47 Tucanae. The researchers’ simulations using tidal effects and the proximity of nearby stars showed that hot Jupiters would be unlikely to survive even when metallicity is left out of the equation. In fact, Debes and Jackson have found that approximately one third of hot Jupiters won’t survive the first billion years of a cluster’s life, not to mention the eleven billion years 47 Tucanae has been in existence. The simulations suggest that at 47 Tuc’s age, at least 96 percent of the hot Jupiters would have perished.
Here Kepler becomes a relevant tool. The mission has four open clusters (much less dense than globular clusters) in its survey field, in a range of ages from less than half a billion to nearly 8 billion years old. All of these clusters appear to have the raw materials from which planets are formed, making them a good test for the tidal decay model. The new work suggests that Kepler should find up to three times more Jupiter-class planets in the youngest cluster than in the oldest one. Fewer expected hot Jupiters, in other words, as cluster age increases and, as a corollary, increasingly tight orbits for detected planets. We should have some answers soon.
And this is interesting, from the paper on this work, on the results of a planet being consumed by its star:
If planets are engulfed, one would expect a signature of pollution in the stellar atmosphere… or an increase in stellar rotation rates… If the process strips just the envelope but leaves a dense core, there might be an excess of ?5-10 ME planets with short periods above that expected through orbital migration alone. There might even be a mass-period relationship for the remnant cores, analogous to that observed for tidally-stripped white dwarfs… Although transit surveys may have difficulty detecting these small stranded remnant cores, their detection would provide an important clue to the fate of close-in gaseous planets.
As to other planets in globular clusters, we’re forced to look for smaller worlds in far more distant orbits. Says Debes: “The big, obvious planets may be gone, so we’ll have to look for smaller, more distant planets. That means we will have to look for a much longer time at large numbers of stars and use instruments that are sensitive enough to detect these fainter planets.” Personally, I hope we start finding them, if only to validate those spectacular sky scenes I’ve long imagined.
The paper is Debes and Jackson, “Too Little, Too Late: How the Tidal Evolution of Hot Jupiters affects Transit Surveys of Clusters,” accepted by The Astrophysical Journal (preprint).
If tidal lag really was causing orbital decay among some stars in Kepler’s current field of view would Kepler be able to detect the changes in transit time or period before total destruction?
Also do you think a hot jupiter crashing into its star in local clusters would be detectable from earth using small optical (or even TVRO radio dishes?) Has this ever been accomplished. Could some “local” GRBs be caused by hot jupiter impacts?
Thanks very much for this latest series and links. very interesting indeed!
It would be good to get a handle on what the planetary population is like in clusters. To my knowledge there are only 3 known planets in open clusters: one known planet each in the clusters NGC 4349 and NGC 2423, and another around Epsilon Tauri in the Hyades. All three clusters are less than a billion years old. There is also Iota Horologii (HR 810) which is apparently an ejected former member of the Hyades. In globular clusters there is the planet around the pulsar/white-dwarf binary PSR B1620-26 (note that pulsars are probably the best bet for finding planets in globular clusters). All these planets are giant planets on long-period orbits.
One of the more intriguing open clusters (unfortunately not in the Kepler field) is M67, which appears to be of roughly the same age and metallicity as our Sun. I wonder what the prospects are like for habitable planets in the M67 cluster, as it would definitely be interesting to compare a variety of Earthlike planets of the same age. Furthermore, if you’re that way inclined, old open clusters might represent one of the best opportunities for SETI, with many stars of the right age to host mature biospheres in the same part of the sky.
I too have [tried] to envisioned such a star field. Would there ever be night?
In this sense, might be good to talk a little here in terms of mean distance between stars? How close do stars need to be to perturb planets? Clearly the number is smaller than about 4LY, but how close?
It’s a great time to be alive with all of these new things happening! But truthfully I’d jump in a time machine in an instant to get answers to some of the questions discussed here…
Michael, just found a page with some speculations on what that night sky would look like. Have a look; it’s interesting:
http://www.iac.es/gabinete/iacnoticias/winter98/xplaneta.htm
As for minimum distance for perturbation, I’m not aware of the number, but maybe some of the readers know.
@andy;
yes, M67 is facinating in that respect, being almost a sold and about as metallic as our sun. And containing a high proportion of sunlike stars (over 100, out of a total population of about 500).
Other interesting open star clusters are NGC 188 (at about 5400 ly, twice as distant as M67), which is about 5 gy old and about same (?) metallicity as our sun, and NGC 6791 (at about 13,300 ly, 5 times as distant as M67) which is about 7 to 8 gy old, the oldest known star cluster in our Milky Way galaxy and yet with over twice as high metallicity as our sun.
Correction: I said NGC 6791 is “the oldest known star cluster in our Milky Way galaxy”. That should have been open star cluster. Some globular clusters, such as 47 Tucanae, M13 and Omega Centauri are much older.
Michael Spencer: what do you mean by perturbation? Bear in mind the gravitational force has (to the best of our knowledge) infinite range. And the magnitude of perturbation is going to depend on the mass of the star and the velocity of the encounter.
Thanks, Paul. Some quite qualified comments for sure on that site. I think I’ll drag Asimov down from the shelf and have a look at Nightfall.
It’s quite reassuring to have our finest minds flummoxed by the wonder of that sky, wouldn’t you agree?
A bit further to Michael Spencer and andy: even if the average distance within such a star cluster is o.1 ly, resulting in a star density several thousand times higher than in our part of the MW (where it is about 3 ly), that is still an enormous distance and probably more than enough to allow for long-term stable planetary orbits. After all, planetary systems have been detected for medium-close binaries with minimum separations of only a few tens of AU.
Andy and Ronald,
I mean a star close enough to disrupt an orderly and stable system. It’s postulated that the never observed Oort Cloud may extend about 1LY from Sol, and if this ‘cloud’ actually exists, comets may have periodically pelted the eight planets. The movement of these comets would be the result of Alpha Centauri’s effect, as it is presently postulated.
On the infinite range of gravity, surely Andy recognizes that, after a certain distance, there is no practical effect? What that distance might be, I don’t know, but of course the principal is well observed.
On a far more entertaining conversation: At a distance of .1LY, or about 5000 AU, things obviously become much more dangerous.
Yes, there are detected orbits with separations of ‘tens of AU’. I don’t think I have heard of these as being characterized as stable or not. Imagine another star between Neptune, at 30 AU±, and Saturn, at 9 AU ±. As I understand theories explaining the evolution of our solar system, an object with the mass of a star in that vicinity surely would not allow stable orbits of 1AU or so? Someone smarter than me will calculate this I am sure.
In any case, what fun to speculate!
Michael, agreed, the speculation is a great pleasure. Re stable orbits, it’s been pretty well established that there are stable orbits for planets in the Alpha Centauri system out to 2-3 AU, though no farther — it must be added that we don’t know whether there are any planets there, and there seem to be real issues about planetary formation. But the orbits seem stable even in this close binary.
Michael Spencer: that then raises the question of how to define an “orderly and stable” system, and what criteria are for disrupting it. For example, the planetary orbits in our own system are unstable, though it may be billions of years before the instability does anything dramatic like ejecting a planet. Furthermore the system exhibits chaotic behaviour: very small changes to the conditions result in very different outcomes, hence the relevance of the infinite range of the gravitational force. It would be impossible to determine for sure whether an encounter had introduced, say, an instability that would lead to ejection of a planet after a billion years or so, yet this would presumably represent disruption of a system that would otherwise have lasted for a much longer time if it were in isolation.
But hey, you said yourself you don’t find this kind of thing entertaining anyway. What fun to speculate, indeed?
My speculation would be that the relevant factor here is not average distance, but the (of course related) frequency of near collisions. How often does a star get approached within, say 10 AU, by another star? Such close encounters can probably be assumed to disrupt a planetary system. The average lifetime of planetary systems will then be similar to the average time between near collisions for a given star. I recall reading that this near-collision frequency is a known quantity in clusters, but I may be wrong.
What I did not get in the post is the link between the “tidal effects upon planets much closer to their star than Mercury” and being in a cluster. Is that tidal effect supposed to be different in clusters?
I wrote in the thread on Poul Anderson’s Answer to Fermi (https://centauri-dreams.org/?p=14157&cpage=1#comments), that wide sunlike binaries (rare though!), such as Zeta 1 and 2 Reticuli and 16 Cygni A and B, would be very conducive, stimulating and tempting with regard to interstellar travel for any existing civilization there: another sunlike star with habitable or easily terraformable planets so relatively nearby (about 6000 AU or 0.1 ly for Zeta 1 and 2 Reticuli, about 850 AU or 0.013 ly for 16 Cygni A and B).
Andy’s comment on M67 makes me realize that this would be even much more the case for any civilization in such a star cluster: star and planetary system hopping would be feasible even with ‘ordinary’ nuclear propulsion. Talking about an envelope or bubble of expansion!
There is a paper somewhere (wish I could keep better records of these things !) which contends that a great many terrestrial planets (about 80 per star) would be made in these lower metallicity clusters at the expense of giant planet formation. Many of these terrestrial planets get ejected from their natal systems by close encounters and form a kind of delocalised floating planet population within the cluster. Others in closer orbits are quite stable. Possibly this theory has now been discounted via microlensing surveys?
This is an interesting if rather old paper (2001) : Free Floating Planets Not So Surprising. 50-100 planets formed per star in these low-metallicity systems, and gas giant formation is rare because the protostellar disc is truncated at low radius. Many planets in stellar clusters are liberated from their natal systems by close encounters. In a globular cluster, a large fraction of these liberated planets do not have the escape velocity of the cluster:
http://arxiv.org/PS_cache/astro-ph/pdf/0108/0108350v1.pdf
So does this mean when the first Arecibo Message arrives at Messier 13 in 25,000 years time there may be no one there to detect it due to a dearth of planets?
http://www.seds.org/messier/more/m013_msg.html
Of course living in a globular star cluster might be hazardous to one’s mental health anyway:
http://escapepod.org/2007/04/05/ep100-nightfall/
Then again, advanced ETI might find all those stars bunched up together a wonderful galactic resource trove:
http://www.aeiveos.com:8080/~bradbury/MatrioshkaBrains/GCaA.html
I think it is worth emphasising the paper does not at all discount the possibility of terrestrial planets in globular clusters.
Also all stars start life in clusters of some sort: when the solar system was young the sun was part of a cluster and the local stellar density would probably have been many times what it is now. Yet we are here.
This paper concentrates on explaining the lack of hot Jupiters in this globular cluster by their tidal destruction. They say that no other explanation is necessary. Well who says this is “the” explanation ? I’m no expert but I have read other papers on this topic. A very credible alternative explanation is that they are not formed in the first place. This is due to the low metallicity, coupled with the protostellar disc being truncated in dense clusters outside the snow line. This explanation allows for small terrestrial planet formation close in to the star, indeed a great many are formed according to some models.
Ref. to andy’s and my own comments on M67, this news just in:
Swedish and Canadian astronomers have discovered a star in M67, M67-1194, that is the closest solar twin discovered so far (closest not in distance, of course, but in similarity to our sun).
Reference: http://arxiv.org/abs/1009.4579
Most fascinating I find the implications for formation and development of solar type stars, suggesting that sunlike stars probably (often) originate in such open clusters and then (usually) migrate out of the cluster.