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Astronomers have potentially confirmed a five-planet exoplanet system around an 11-billion-year-old star in our galaxy. The star, Kepler-444 (a.k.a. HIP 94931 in the constellation Lyra), is a K0V star, meaning it’s a red dwarf slightly cooler than our Sun that’s fusing hydrogen in its core.
Tiago Campante (University of Birmingham, UK, and Aarhus University, Denmark) and colleagues investigated the five repeating planet-like transit signals, which were caught by the Kepler spacecraft over the course of the four years of the craft’s observations. With estimated sizes ranging from 0.4 to 0.74 times Earth, all five should be rocky, but not a one is in the star’s habitable zone: they all orbit the parent star within 0.08 Earth-Sun distances, or less than one-fifth the size of Mercury’s orbit. An optimistic habitable zone would start at 0.47 Earth-Sun distances, nearly six times farther out.
Kepler-444 is not the first star of this age with planetary children: astronomers have also found two planets around Kepler-10 (about 10 billion years old) and two planets around Kapteyn’s star (also around 11 billion years old). Those planets are larger (super-Earth class or a bit bigger). But as the press release from Yale explains, it’s exciting to find exoplanets that formed around the same time as the Milky Way itself was coalescing.
You can read more in the team’s paper, which appears in the February 1st Astrophysical Journal. (Preprint draft here on the arXiv.) There's also a neat animation of the Kepler-444 system (comparison with our solar system begins at 0:35 time stamp).
Comments
Tom Hoffelder
January 31, 2015 at 9:57 am
Interesting, "all five should be rocky." A star that old must be Pop II? Apparently there were some Pop III giants?
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Jim-Baughman
January 31, 2015 at 11:37 am
From what I recall reading recently in S&T, Pop III stars were mega-giants, which could contain thousands of solar masses without going supernova, because being composed entirely of hydrogen their dynamics were vastly different from today's stars with their high metallicity, and would not even begin functioning as stars (i.e. igniting fusion reactions) until they had achieved enormous size and mass. So these stars had the potential for distributing rocky material once they exploded, but your point is a good one, suggesting there must have been vast numbers of Pop III stars to have salted the universe with so much rocky matter as to allow such as system as Kepler-444 to form.
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Jim-Baughman
February 1, 2015 at 9:22 pm
Yes, there were Pop III giants, which attained much larger sizes than are possible now because they were composed entirely of hydrogen so the supernova dynamics were different, Stars with higher metallicity cannot attain such huge sizes as these first Pop III stars. But these stars did go supernova (or perhaps even hypernova) and thus were able to scatter elements into the universe from which rocky planets could form. I believe this topic was covered in Sky and Telescope about a year ago.
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Philip -Caputo
February 3, 2015 at 12:28 am
This discovery strongly indicates, if it does not prove, that planet formation in our galaxy occurred much earlier than previously thought. This raises an intriguing possibility. The Kepler solar system was 7 billion yrs. old when our system was formed, certainly more than enough time for life to have established itself on at least one of the planets, and then to have evolved. Is this a correct supposition, or am I indulging in sci-fi fantasy?
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Jim-Baughman
January 31, 2015 at 11:43 am
Also, I am curious that so many of the planetary systems discovered in the last few years contain planets that orbit extremely close to their stars. Is it just that these compact systems are easy to detect, so they are found while it would take much longer observation to detect a system like ours? Kepler can only have assessed these systems for relatively short periods of time, making the detection of planets such as Jupiter, with its 12-year orbit around the sun, outside Kepler's capability to detect.
Am I missing something?
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Tom Hoffelder
January 31, 2015 at 3:41 pm
https://en.wikipedia.org/wiki/Kepler_(spacecraft)#Objectives_and_methods
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February 1, 2015 at 6:16 pm
Jim-Baughman - the transit method of planetary detection has a strong bias towards finding planets in small orbits. While a planet with an Earth-like orbit around a Sun-like star has only about a 0.5% chance of producing an observable transit, a planet like Kepler 444b with an orbital radius of only 0.04 AU, for example, has about a 12% chance of having its orbit aligned to generate an observable transit. Also, Kepler was only observing the same patch of sky for four years. Coupled with the fact that three transits need to be observed to constitute a planetary "detection", only planets with orbital periods less than 16 months can be firmly established using Kepler data. That corresponds to a maximum orbital radius of no more than ~1.2 or ~0.6 AU for a 1.0 or 0.1 solar mass star, respectively. As a result, Kepler is much better at finding compact planetary systems than more spread out systems like our own. The precision radial velocity (RV) method of planetary detection (which have been used to indirectly detect most of the planets found before NASA's Kepler mission) has its own biases that make finding big planets in small orbits much easier, as well.
That being said, there is evidence to suggest that planetary systems come in a wide variety of types ranging from compact to more spread out with a wide mix of planet sizes from sub-Earth to super-Earth to Neptune to Jupiter-size planets. More detailed statistical analyses that take into account the biases in the Kepler data set along with various RV surveys as well as future results of other missions (e.g. Gaia, TESS, etc.) will help astronomers start to make more definitive statements about the architectures of extrasolar planetary systems and where our Solar System fits into the mix. Already initial results from Kepler indicate that around red dwarfs, at least, gas giants are rare and compact coplanar planetary system exist around about half of these small stars.
http://www.drewexmachina.com/2014/10/24/architecture-of-m-dwarf-planetary-systems/
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