When the search for planets in star-packed globular clusters began, expectations were high. One cluster, 47 Tucanae, was expected to yield at least a dozen planets from around its 34,000 candidate stars, but the search was unsuccessful. More than 450 exoplanets have been found, but most of them have been detected around single stars.
Why are planets not being found in these neighborhoods? Jackson explains that "there are lots of stars to beat up on them and not much for them to eat." The high density of stars increases the chances that a nearby star will affect a planets orbit and potentially kick it out of its solar system. Additionally, surveys of globular clusters have shown that they are rather poor in metals, which are essential for making planets (this is known as low metallicity). In essence, planets in these neighborhoods get their lunch money stolen and get kicked out of town, which is why the search for planets in these areas have come up short.
The research also suggests that "hot Jupiters" are more likely to be found in younger star clusters than older ones. This is because in addition to the problems listed above, the large planets that are very close to the sun could be destroyed by their cramped orbits. Since the planets are at least 3 times closer to their host stars than Mercury is to our sun, the gravitational pull of the planet on the star can create a tide on the star. This bulge on the star is always just a little behind the planet, and its gravity essentially tugs back on the planet, reducing the energy of its orbit. As this energy drops, the planet becomes closer to the star, and this bulge gets even bigger. Eventually, the planet will crash into the star or it will be torn apart by the star's gravity, according to the researcher's model of tidal decay.
Debes and Jackson modeled this effect on a hypothetical 47 Tucanae, and it predicted that most of the planets would have been destroyed, regardless of metallicity. This would explain the unsuccessful search for exoplanets, especially since the model predicts that more than 96% of the hot Jupiters would be destroyed by the time a cluster was as old as 47 Tucanae (11 billion years). The researchers look forward to the results of the Kepler mission, which is searching for hot Jupiters as well as smaller planets, to test their model. If their model is right, finding planets could be getting harder since the large and obvious ones may be long gone.
Taking this all in, star clusters are tough places for planets to hang out. Perhaps the search for exoplanets should focus on younger, higher metallicity, and less dense clusters that haven't bullied their planets just yet.
This is a really interesting article! I don't really know that much about star clusters besides what we've talked about in class but I'm just wondering what kind of star cluster they were basing this research on? And if the different types of star clusters would make a difference in the possibility of the existence of gigantic exoplanets? I started doing a little googling about the different types of star clusters that exist in the universe and it sounds like they're referring to primordial star clusters but I'm just wondering if this also applies to stellar sibling clusters? According to the website that I found, these clusters are much richer in heavy elements and even contain our own Sun! I don't really know if that makes any sense, but its just a thought... Anyway, cool article choice!
ReplyDeleteHi Chandler,
ReplyDeleteit's been known for a while that planetary systems seem to be more common around stars with higher "metallicity" (i.e. higher amounts of heavy elements). Globular clusters like 47 Tuc are known to be metal poor, and not great targets. However, there's another type of star cluster--open clusters in the disk of the Milky Way that are more promising in terms of metallicity. As Galia's question suggests, the type of cluster can strongly affect the metallicity. However, it's also interesting to note that the calculation by Debes and Jackson suggests that stellar systems with hot Jupiters may be younger than average, because of the tidal decay mechanism. This has some consequence for the inferred fraction of solar system-like planetary systems--surveys for current planets will overestimate the fraction of planetary systems that are like our own.