Kepler’s Worlds

posted by Alan Hale on March 1, 2011

Alan Hale writes as a new member to Project Icarus. Alan’s research interests include the search for planets beyond the solar system, including those which might have favorable environments for life; stars like the sun; minor bodies in the solar system, especially comets and near-Earth asteroids; and advocacy of spaceflight. He is primarily known for his work with comets, which has included his discovery of Comet Hale-Bopp in 1995 and his participation in the International HalleyWatch during the return of Halley’s Comet in 1986. This article originally appeared in the Alamogordo (New Mexico) Daily News as part of the weekly column ‘In Our Skies.’ It has been modified slightly from its original version. As recently as two decades ago, when this author was in graduate school and researching this particular topic, the subject of planets outside our solar system was almost completely theoretical. There were quite a few ideas floating around as to what these worlds might be like, and where they might be found, but at that time there were no such worlds that had been discovered and confirmed, and thus the subject remained in the realm of speculation. That state of affairs began to change in the mid-1990s, when our technology reached the point where we could begin detecting these worlds. At first the discovery rate was just a trickle, and each new discovery was greeted with quite a bit of fanfare; as time went by, however, more and more worlds began to be discovered, and nowadays the discoveries are so commonplace that most of them hardly make a splash at all. The current census of worlds outside our solar system stands at approximately 525. It should be remembered that, in most cases, we aren’t detecting the actual planet, but rather the gravitational effects it is having upon its parent star. A large, massive planet is thus more likely to be detected than a small one, and a planet orbiting close to its parent star is more likely to be detected than a world orbiting farther out. For this reason, most of the early discoveries were the so-called “hot Jupiters,” i.e., very large worlds orbiting very closely around their parent stars, although as time has gone by (and our instrumentation and techniques have improved) we’ve been detecting smaller and smaller worlds orbiting farther and farther out. A fraction of the currently-known planets have been discovered via what is known as the “transit” method, i.e., if their orbits around their parent stars happen to lie in or near the same plane as our line of sight to those stars, the planets in question will periodically cross in front of, or “transit,” their parent stars, causing a small but measurable drop in the amount of light we receive from the star. As with the primary method of planet discovery this approach favors larger planets and planets closer in, although not to as significant a degree; meanwhile, this method can, in theory, detect Earth-sized planets utilizing our present technology. The primary drawback to the “transit” method is that we would expect only a small percentage of the planetary systems in the galaxy to be aligned in such a way that would allow the method to be successful in detecting planets. The Kepler mission, launched two years ago in March 2009, is an orbiting telescope that is designed to search for planets using the “transit” method. It gets around the main drawback by continuously examining one relatively small section of the sky; this area contains several hundred thousand stars, and – under the assumption that most stars do have accompanying planets – this in turn would mean that several hundred stars and planetary systems would have the right alignment. The section of sky being monitored by Kepler lies in and near the western “wing” of the constellation Cygnus, the swan, currently visible in the northeastern sky shortly before the beginning of dawn. Almost from the very beginning Kepler has been finding planets, and news about these have been reported from time to time. Within the past month, however, some very dramatic findings from Kepler have been announced, which on one hand seemingly validate our assumption that most stars have accompanying planetary systems, and on the other hand challenge our assumptions as to what these systems might be like. One of these discoveries is a world known as Kepler-10b, which in terms of physical size is only about 40% larger than Earth but which is over 4 1/2 times more massive than Earth, giving it the average density of an iron cannonball. Kepler-10b orbits very closely to its parent star (taking only 20 hours to do so) and is thus extremely hot – hardly a place we would want to visit – but by some criteria it can be considered as the first-known confirmed example of a “terrestrial,” or “solid” planet. Then there is the Kepler-11 system, wherein Kepler has detected as many as six planets, five of which orbit their star at a distance less than that of Mercury’s orbit, and all of which are a few times larger than Earth. Unlike Kepler-10b, these worlds are less dense than Earth, and at least one appears to be a “marshmallow” world that is even less dense than the gaseous planet Saturn. Perhaps the most exciting news from Kepler is the announcement of over 1200 “candidate” planets, i.e., worlds where additional observations are needed to confirm their existence; the astronomers involved estimate that between 80% and 90% of these will turn out to be real. Over 50 of these “candidate” planets orbit around their parent stars in the stars’ respective “habitable zones,” i.e., those distances where the temperatures are such that water can exist as a liquid. And – most exciting of all – of those candidate worlds, about five seem to be about the same size as Earth. We can expect that these five worlds in particular will soon be the objects of intense scrutiny, although since they are located a few hundred light-years away from us detailed studies are not going to be easy. Nevertheless, it is distinctly possible that within the not-too-distant future we may have some pretty solid evidence of one or more potential “Earths” in our galaxy. And our search in only just beginning . . .  

Artists Impression of an Exoplanet. Image courtesy of Adrian Mann.

————————————— Alan Hale is a professional astronomer who resides in Cloudcroft, New Mexico. He is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is

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6 Responses to Kepler’s Worlds

  1. Fascinating indeed, but unfortunately not targets for Icarus. A pity the other wing of Cygnus wasn’t chosen, as it includes the nearby 61 Cygni double star. Any news on that system?

    It will clearly take quite a while before we have a good idea of the range of planetary systems that are possible. I can understand the fascination with finding Earth analogues, but we need to know about all the other worlds, not just those that look like home.

  2. Adam says:

    Hi Alan
    Nice blog-post. The paper on Habitable-Zone Earth analogues and their abundance, at least in Kepler’s field of view, gives me pause…

    …main conclusion being that, using the 0.95-1.37 AU equivalent hab-zone boundaries, there’s ~2 Earth-analogues in every 100 stars does make me wonder how far out “Icarus” needs to venture to find one. Fortunately having Alpha Centauri so close might boost the odds.

  3. Adam, that’s 2% of “Sun-like” stars. What is a “Sun-like” star? A G dwarf? A K, G or F dwarf? What about M dwarfs, which comprise 80% of stars in this part of the Galaxy?

    Another question: how do these planets match up with stars in terms of whether they are single or in multiple star systems? Do stars have planets regardless, or are the planets detected so far biased towards singleton stars or alternatively towards stars in well spaced doubles or triples, like our old friend Alpha Centauri, or 61 Cygni? What about very close binaries like eclipsing binaries: do they have planets orbiting both stars a long way out?

  4. Adam says:

    Hi Stephen
    Good question. My suggestion – read the preprint and understand their statistical method. Will make more sense then.

  5. They define “sunlike” as F G or K stars. Around M class stars, we already have Gliese 581 in our neighbourhood with (possibly) two habitable worlds. As for their statistical method, they extrapolate from a region of certainty with power laws based on observations where Kepler’s signal to noise ratio is higher. Also, there’s no telling whether these worlds have large moons. If large moons are common (as some protoplanet collision simulations suggest) then some super-earths may have a large, habitable moon. That might radically increase the number of potential candidates, especially if there are multiple moons.

  6. One more thing, if the HZ requirements are relaxed, then the probability of a habitable planet around a sunlike start rises to an average of 2.8% with an upper bound of 4.7%. With these (very) loose estimates, the probability of either Alpha Cen stars having a (marginally) habitable planet is (at most) ~9% and more likely 5-6%.

    That being said, the estimate is somewhat of a lower threshold, and is based on power laws. As we have seen with Kepler 11b, that may not hold true. It also depends on the “snapshot” of high-accuracy planets holding true. If either assumption is violated, then it is likely that estimates would have to be revised upwards.

    Kepler was designed on the thumb-suck estimate of finding at least one or two earth analogue planets. It has found five candidates already. So all things considered, earth analogue planets are at least 2 or 3x more common than initially hoped. A 1 in 50 chance of finding a planet, if you assume 1 “sunlike” star within 1000 cubic lightyears, makes the average earth analogue a neighbourly 23 lighyears away.

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