By Dr. Franck Marchis
Astronomer at the Carl Sagan Center for the Study of Life in the Universe, SETI Institute
It is done. The Kepler team finally announced the discovery of its first terrestrial exoplanet. A referred journal, accepted in the Astrophysical Journal by Natalie Batalha and a large number of colleagues, describes this new member of the exoplanet family. This is the 519th known exoplanet based on the Extra-solar Planets Catalog, but definitely a special one.
This new exoplanet, named Kepler-10b, orbits around a Sun-like star, twice the age of our sun (~12 Gyrs) but similar in size and color. This main-sequence star is identified as KIC 11904151, one of the 13.2 million stars of the Kepler Input Catalog and it is located at 173 pc (576 light-years) from us. Thanks to the Kepler accurate photometric capabilities and its automatic pipeline module, a small and periodic (P~20h) attenuation of the star lasting 1.8 h and with an intensity of 0.015% was detected on a regular basis starting from May 13, 2009. Several hundred detections were detected. A second, long-period (45h) and more intense (0.038%), attenuation was also seen 6 times by manual inspection starting on May 19, 2009.
Additional observations from ground-based telescopes using high resolution spectroscopy and high-angular resolution provided by speckle and adaptive optics techniques allow the team to confirm that these attenuations are indeed due to the presence of exoplanets orbiting around this star.
Doppler effect (wobbling of the star) with an amplitude of ~3 m/s due to the closer planet was detected using the W.M. Keck II telescope from observations collected in May-August 2009. These remarkably accurate data confirmed the genuineness of the closer planet and allow constraining the mass of the planet. At the American Astronomical Society (AAS) meeting on January 10, 2011, the team announced the discovery of Kepler-10b, an Earth-like exoplanet 5 times more massive than Earth, 1.5 larger than Earth and most likely made of rocky/iron material since its density is between 6 and 11 g/cc.
The second transit name KOI-72.02 has not yet been confirmed by radial velocity, most likely because the exoplanet candidate is too far and/or too small. Its mass should be less than 20 Earth Mass, it is twice the size of Earth and orbits at 0.24 AU from its sun, 2/3 the distance of Mercury around our Sun.
For the sake of completeness and to give a broad context, it is not the first time that a rocky exoplanet has been claimed to be discovered. I mentioned in one of my previous posts the detection of Corot-7b (R~1.6 x Rearth; M ~ 5 x Mearth), discovered by transit in 2009 using the COROT mission, which has a bulk density from 0 to 11 g/cc. GJ1214b is another example of transiting super-Earth planet, almost 3 time larger than Earth and with a density of 1.9 g/cc. Small, but exotic exoplanets, were also discovered around the pulsar PSR B1257+12 with a mass of less than four time Earth Mass.
The figure above was presented by Natalie Batalha at the press conference of the 217th AAS at Seattle, WA. The uncertainty in the density of Corot-7b is quite large, so it is impossible to know if COROT-7b is a water planet, a rocky planet or an iron-core planet. The measurement of density of Kepler-10b is obviously more accurate, implying that the exoplanet has a composition similar to Earth or an iron-core planet. Kepler-10b is the first unambiguous detection of a rocky exoplanet, and the first super-Earth discovered around a sun-like star. This is obviously a significant step in the search for exo-worlds and for the Kepler team, and also a major achievement since it was the main objective of this mission.
What's next?â¨ It is remarkable that a few days of observations after the commissioning phase of the telescope (which ended May 12, 2009) already permitted the detection of this first rocky planet. Today we have one unambiguous Earth-like exoplanet, but how many of them could we expect to find after a year of survey which includes several hundred thousands of stars? I am guessing we will get a response in a few weeks or months.
Franck Marchis @AllPlanets on Twitter
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You can easily calculate its lifetime using the critical mass divided by the host stars latitude multiplied by the cosmic characteristics of the sequence. As an example- Kepler-10 star mass is 0.895 x Msun would calculate its lifetime in the MS!
Given the age of the star -you've givem 12GY -I saw 8GY elsewhere, but in either case it was formed during an epoch when mettalicity would have been expected to be low. I guess the impression I had, that maybe only more recently formed stars had enough heavy elements to form sizable rocky planets must be wrong? Any thoughts on this issue.
The star is being advertised as similar to the sun, yet a solar mass star of that age would be leaving the main sequence -or have already ended its active lifetime, so I presume this star is actually less massive then the sun. But maybe it is close to the transition from the main sequence?
The age of the star is poorly known since it is estimated to 11.9 +/-4.5 Gyr. The characteristics of the host star, and how they determined it, are described on page 47 of the refereed paper.
A G2 star like the sun will remain in the main sequence for 10 Gyrs. Kepler-10 star mass is 0.895 x Msun you can calculate its lifetime in the MS using the following relation:
1E10 *(Msar/Msun)^(-2.5) = 13.2 Gyrs. Consequently, an estimated age between 7.4 and 16.4 Gyrs is not incompatible.
Minor matters like the age of the universe notwithstanding...
Thanks, frank. I didn't try linking the paper (I almost never have permission to read more than the abstract, then there is the issue of time). In any case the rate of change in luminosity increases during the main sequence phase. Do SETI types consider late stage main sequence stars to be brightening too rapidly to be of interest?
Do you have any comments on the mettalicity versus rocky planets (and possibly the size of them) issue?
About the metallicity:
The [Fe/H] is estimated to -0.09 +/- 0.04 (see http://en.wikipedia.org/wiki/Metallicity for a definition of the metallicity).
Fisher et al. (2005) published (see Fig. 6 of http://astro.berkeley.edu/~gmarcy/marcy_japan.pdf) an histogram showing the occurrence of exoplanets vs iron abundance [Fe/H] of the host star measured spectroscopically.You can see that the occurrence seems to increase significantly when [Fe/H] > 0.10, this exoplanet could be an anomaly. However, one of the goals of Kepler is to improve the statistic on the formation rate of exoplanets to confirm this relationship and refine it taking into account as well the spectral type/mass of the stars. More soon on this topic when Kepler large catalog will be released.
Is Kepler-10 really in the constellation Cygnus? Judging by its coordinates, it seems to me that it's just beyond the border to Draco. Did I get that wrong?
"...orbits around a Sun-like star, twice the age of our sun (~12 Gyrs) but similar in size and color"
An estimated age of at least 7.4 Gyr means most radioactive minerals in the mantle and core will have decayed. So, even without the extreme heat from the star, tectonic activity would be slowing down making the planet adverse to life.
Even if the star has other planets at more benign distances, they are likely tectonically dead, with eroded surfaces and little chemical weathering. Thus, the temperature of an extant biosphere will not self-regulate anymore.
The promising thing about the system is -as mentioned- that even old systems can form terrestrial planets. Lots of lichen and bacteria out there :)
In my first draft I labeled it in the constellation of Draco but I checked on the IAU official page and it seems to be in Cygnus even if it is very close to the edge. I am on my cell phone right now so I can compare wikipedia with the official IAU page.