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)
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)
Reference:
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]