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Kepler-101: a reversed planetary system observed with HARPS-N

Thanks to forty precise radial velocities obtained with the HARPS-N spectrograph at Telescopio Nazionale Galileo (TNG) together with the Kepler photometry, and in the context of the GTO program, astronomers were able to characterize an anomalous planetary system: Kepler-101

They found that this system consists of a hot super-Neptune, named Kepler-101b at a distance of 0.047 AU from the host star (a slightly evolved and metal-rich G-type star), and an outer Earth-sized planet, named Kepler-101c, with semi-major axis of 0.068 AU and mass less than four Earth masses.

Kepler-101b and Kepler-101c were first discovered by space born telescope Kepler thanks to the transit method, where the star light is dimmed when the planet passes (transits) in front of the observed star disc (Figure 1).
These observations made possible for the Kepler team to determine dimensions and orbital periods for the two planets, but not their masses, which are a crucial piece of information in order to understand their internal characteristics.

The new combined spectroscopic and photometric analysis (Figure 2) allowed to derive a dynamical mass for Kepler-101b and to place constraints on that of Kepler-101c. The much improved characterization of the Kepler-101 system also permitted to identify the first fully-characterized super-Neptune planet: Kepler-101b joins the rare, known, transiting planets in the transition region between Saturn-like and Neptune-like planets in terms of mass and size. It also shows an internal structure with a significant fraction of heavy elements (more than 60% of its total mass).

This study also provides the first observational evidence of a multi-planet architecture with close-in low-mass giants and outer Earth-sized objects. “Actually, one of the main results of this work is that the architecture of the Kepler-101 planetary system, containing a close-in giant planet and an outer Earth-sized planet does not follow the trend (found in the majority of Kepler systems of planet pairs) that the larger planet has the longer period, like in our own solar system” explains Aldo Bonomo, first author of this paper, a joint work of the Harps-N Consortium collaboration (Italy, Switzerland, United States and United Kingdom).

In particular, as both planets are observed transiting their parent star, astronomers suggest that these planets evolved and migrated toward their star through disc-planet interactions, rather than through dynamical interactions. It is also probable that the larger super-Neptune formed farther away from the star and that only afterwards could overcome the smaller one without damaging it.
Emilio Molinari, TNG director, adds that “such conclusions are possible only thanks to the precise radial velocity measurements of our Harps-n spectrograph”.

Paolo Vettolani, Scientific Director of INAF is proud to say that “these planets keeps surprising astronomers which are studying the planets of other solar systems. We are just now uncovering the geography of planetary systems, trying to understand their history".

Figure 1: Photometric observations from spacecraft Kepler showing the transits of the super-Neptune Kepler-101b (left) and of the Earth-sized Kepler-101c (right). Red lines show the best fit model of the transits.

Figure 2: phase-folded radial-velocity curve of Kepler-101 star which are the signature of the velocity variation of the star along the line of sight due to the perturbations of the more massive planet Kepler-101b. The superimposed, black solid line, shows he Keplerian orbit model from TNG data.

Figure 3: artist impression of the Kepler 101 planetary system. Credits: TNG/ V. Guido

For more information:

Characterization of the Kepler-101 planetary system with HARPS-N
Aldo Bonomo, INAF-Osservatorio di Torino,

TNG & Harps-N
Gloria Andreuzzi, INAF-TNG,