Figure 2: Artist’s impression of a rocky exoplanet.
Over the years, observations have shown that small planets are common around stars, regardless of whether the star is metal-poor or metal-rich. In contrast, gas giant planets tend to be more abundant only around metal-rich stars. Kepler-444 is a planetary system that is home to five small rocky planets that orbit around a metal-poor Sun-like star. The host star of this planetary system is estimated to have formed 11.2 ± 1.0 billion years ago, when the Universe was less than 20 percent its current age. This makes Kepler-444 the oldest known planetary system hosting rocky planets and it is a good indication that planets, especially small rocky planets, can readily form throughout most of the Universe’s 13.8 billion year history. Interestingly, at the time when Earth formed, Kepler-444 and its system of rocky planets were already older than Earth is today.
Kepler-444 is located at a distance of approximately 115 light years away and it is a highly compact planetary system consisting of five transiting, sub-Earth-size planets. The orbit of the outermost planet is only one-fifth the size of Mercury’s orbit around the Sun. All five rocky planets orbit their host star in less than 10 days, and have sizes between that of Mercury and Venus. The radii of the five rocky planets are 0.403, 0.497, 0.530, 0.546, and 0.741 times the radius of Earth; and their orbital periods are 3.600, 4.546, 6.189, 7.743, and 9.740 days; respectively. The host star of this planetary system has 0.758 ± 0.043 times the mass and 0.752 ± 0.014 times the radius of the Sun. Additionally, the star’s effective surface temperature is 5046 ± 74 K and it is a metal-poor star with less than one-third the Sun’s metallicity.
Figure 2: Semi-major axes of planets belonging to the highly-compact multiple-planet systems Kepler-444, Kepler-11, Kepler-32, Kepler-33, and Kepler-80. Semi-major axes of planets in the Solar System are shown for comparison. The vertical dotted line marks the semi-major axis of Mercury. Symbol size is proportional to planetary radius. Note that all planets in the Kepler-444 system are interior to the orbit of the innermost planet in the Kepler-11 system, the prototype of this class of highly-compact multiple-planet systems. Campante et al. (2015)
Figure 3: Sizes of the five rocky planets of Kepler-444 in comparison with the Moon, Mercury, Mars and Earth. Image credit: NASA.
Campante et al. (2015), “An ancient extrasolar system with five sub-Earth-size planets”, arXiv:1501.06227 [astro-ph.EP]