EMBARGOED UNTIL: 1:00 a Sometimes in science, coming up empty can be just as important as finding something. Good news for a team of researchers who, using the
Hubble Space Telescope, could not find a single extrasolar planet in a systematic search of a patch of distant sky absolutely loaded with stars.The results of the study -- zero-for-35,000 -- mean that planet formation may play by different rules in different parts of the galaxy, an idea that is likely to spur a whole new line of research.
"This could be the first tantalizing evidence that conditions for planet formation and evolution may be fundamentally different elsewhere in the galaxy," said Mario Livio of the Space Telescope Science Institute in Baltimore, Maryland.
Researchers used Hubble's keen eye to peer into the heart of a "globular cluster" of stars known as 47 Tucanae, a place 12,000 light-years away where more than a million stars congregate. With all those stars, one might expect an orbiting planet or two -- especially with the
count of known extrasolar planets, or exoplanets, growing beyond four dozen.Most of the exoplanets found so far haven't been more than 50 or 100 light-years away. (A light-year is the distance light travels in a vacuum in one year, about 5.88 trillion miles or 9.46 trillion kilometers). But Hubble can see farther, and with greater clarity, than the ground-based telescopes used for most searches.
Yet 47 Tucanae, the most distant region ever searched so carefully for planets, was a dubious target to begin with.
Its stars are much older than those nearer our own
Sun, and they are low on heavier elements often considered necessary ingredients for cooking up planets. On top of that, the tremendous gravity and energy associated with so many stars packed so close together could destroy fledgling planets before they form, or toss them into their host star for a fiery death, or tug them far away from their origins where they'd be harder to find.So why look there?
Looking in a densely packed region of the sky makes sense, said Morris Aizenman of the National Science Foundation (NSF), both because Hubble's high resolution means it is well suited for such a task, and because statistically one would expect the sheer number of stars to yield a planet.
But with the hunt bagging no results, won't exoplanet hunters be frustrated? To the contrary, Aizenman expects the lack of planets to provoke a great deal more study.
"I think there's no doubt that [this study] is going to increase the impetus to keep looking, and looking in different regions under different physical conditions, to see what kind of objects actually have planets around them," said Aizenman, senior science associate in the NSF's Mathematical and Physical Science Directorate.
The next logical next step, Aizenman added, would be to study another globular cluster to confirm the results. The cluster 47 Tucanae is very old, he said, and probably formed before our galaxy had developed its general shape. "Maybe planetary formation didn't take place at such an early time."
If this is true, then
life as we know it may be a relatively recent development.The painstaking search
Planets as far away as 47 Tucanae cannot be seen directly, so the hunt for them turns creative.
More than 1,300 images were produced by Hubble over an eight-day period. Each of 35,000 stars in the images were then studied for the telltale sign of dimmed light, which would indicate a large planet passing in front of the star.
Based on knowledge of other stars with planets, as well as the planets that have been found using this method, Ron Gilliland of the Space Telescope Science Institute and his colleagues expected that one out of 1,000 stars in 47 Tucanae should have a large planet in close orbit around it, moving across the star's face every five days or less.
By that measure, they should have found 17 large planets -- all at least slightly larger than Jupiter. Smaller planets would not have been detectable.
Instead, they found none, which Gilliland said indicates that this type of planet -- large, close to its star and rapidly rotating -- is not as common everywhere in the galaxy as in our nearby corner.
"We were disappointed to some extent, although we realized all along that this was a potential outcome of the observations," Gilliland told SPACE.com. "Since we succeeded technically, in the sense that we are confident we reached the sensitivity to see the planets if they are there, we are pleased that the observations resulted in a still significant finding."
Gilliland worked on the study with Tim Brown of the National Center for Atmospheric Research and two dozen other scientists. The results were announced October 30.
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