Friday, December 18, 2015

A Small Planet with Giant Companions

Figure 1: Artist’s impression of a Jupiter-like planet with a moon in orbit around it.

Kepler-454b is an exoplanet that orbits a Sun-like star. It was previously detected by NASA’s Kepler space telescope which hunts for planets by looking for the dip in a star’s brightness when a planet passes in front of it. Kepler-454b is estimated to have 2.37 ± 0.13 times the diameter of Earth based on how much light the planet blocks when it transits its host star. Additionally, the frequency of the transits indicates that Kepler-454b has an orbital period of 10.6 days.

Using 63 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 36 radial velocity observations obtained with the HIRES spectrograph at Keck Observatory, Gettel et al. (2015) show that Kepler-454b has 6.8 ± 1.4 times the mass of Earth. Furthermore, the radial velocity measurements also indicate the presence of two additional non-transiting companions.

Figure 2: Transit light curve indicating the presence of Kepler-454b. Gettel et al. (2015)

 Figure 3: Radial velocity measurements from HARPS-N (blue circles) and HIRES (red squares) indicating the presence of Kepler-454b. Gettel et al. (2015)

One companion, identified as Kepler-454c, is a Jupiter-like planet with at least 4.46 ± 0.12 times the mass of Jupiter in a nearly circular 524 day orbit. The other companion, identified as Kepler-454d, is a massive object with at least 12.1 times the mass of Jupiter and an orbital period of over 10 years. Determining the properties of Kepler-454d is difficult as its orbital period is much longer than the timescale over which the radial velocity measurements were performed. Kepler-454d is quite likely a brown dwarf.

Mass measurements of exoplanets that are smaller than 2.7 times the diameter of Earth show that they appear to fall into two populations. Those with less than 1.6 times the Earth’s diameter follow an Earth-like composition and those with more than 1.6 times Earth’s diameter contain a significant fraction of volatiles. With a density of 2.76 ± 0.73 g/cm³, Kepler-454b is intermediate between a rocky Earth-like world and a Neptune-like world. It falls into the category of planets that are expected to contain a significant of volatiles and/or hydrogen and helium gas.

Figure 4: Mass-radius diagram for planets with less than 2.7 times the Earth’s diameter and with masses measured to better than 20 percent precision. Gettel et al. (2015)

Gettel et al. (2015), “The Kepler-454 System: A Small, Not-rocky Inner Planet, a Jovian World, and a Distant Companion”, arXiv:1511.09097 [astro-ph.EP]