Gandolfi et al. (2014) report on the discovery of a
half-Jupiter mass planet transiting an old Sun-like star every 2.7 days. This
discovery combines data collected by NASA’s Kepler space telescope from 13 May
2009 to 11 May 2013 with spectroscopic follow-up observations performed with
the FIES spectrograph at the Nordic Optical Telescope in La Palma, Spain. Photometric
data from Kepler indicates how much starlight is blocked when the planet transits
in front of its host star, allowing the size of the planet to be estimated. The
FIES spectrograph measures the amount of gravitational tugging the planet has
on its host star and provides the estimated mass of the planet.
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Figure 1: Artists’ illustration of a hot-Jupiter orbiting a Sun-like
star. Image credit: Haven Giguere & Nikku Madhusudhan.
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Figure 2: Phase-folded transit light curve of KOI-183b
showing the best fitting model and residuals. Gandolfi et al. (2014).
%2B-%2B3.png)
Figure 3: Radial velocity data from the FIES spectrograph
with the median, 68th and 99th percentile limits. Gandolfi et al. (2014).
The planet, identified as KOI-183b, is estimated to have
0.595 ± 0.081 times the mass of Jupiter and 1.192 ± 0.052 times the radius of
Jupiter. Given the mass and size of the planet, its bulk density is 0.459 ± 0.083
g/cm³. KOI-183b orbits its host star at a distance of only ~1/28th the
Earth-Sun distance. As a result, KOI-183b is intensely heated and is classified
as a hot-Jupiter. The radius of KOI-183b is consistent with theoretical models
for heavily irradiated coreless gas-giant planets. Being so near to its host
star, temperatures on KOI-183b can reach ~2000 K, hot enough to melt titanium
metal.
Data from Kepler also indicates that KOI-183b periodically
passes behind its host star in what is known as a secondary eclipse. The
secondary eclipse signal has a depth of 14.2 ± 6.6 ppm. From the depth of its
secondary eclipse signal, KOI-183b is estimated to have a very low Bond albedo
of only 0.037 ± 0.019, making it one of the “darkest” gas-giant planets known
so far. Basically, KOI-183b reflects only ~4 percent of the incoming radiation
from its host star back into space. For comparison, that is darker than coal.
Other hot-Jupiters with similarly low Bond albedos include TrES-2b and Kepler-77b.
Reference:
Gandolfi et al. (2014), “KOI-183b: a half-Jupiter mass
planet transiting a very old solar-like star”, arXiv:1409.8245 [astro-ph.EP]