Sunday, July 10, 2016

HIP 41378: A Compact Planet Sampler

Figure 1. Candidate planets of HIP 41378 represented at their relative sizes (radii from Vanderburg et al. 2016). Planets b and c are the only objects with formal validation. The orbital periods of the three outer candidates remain uncertain, and the discovery team provided no data on semimajor axes. Nor could they establish whether the period of candidate e is longer or shorter than that of candidate d.

The ongoing K2 Mission can’t compare with the original Kepler Mission in terms of the sheer number of planet candidates identified. Nevertheless, some K2 discoveries rival the astrophysical interest of Kepler’s crown jewels. Last year’s game-changing results from WASP-47 have been discussed a few times in this blog. A few weeks ago the world received advance notice of another amazing K2 find: a rich, high-multiplicity, mixed-mass system of five planets transiting HIP 41378, described in a recent preprint by Andrew Vanderburg & colleagues (Figure 1).

As the discovery team notes at the outset, “K2 is not as sensitive to planetary systems with complex architectures as the original Kepler mission” (Vanderburg et al. 2016). That’s because K2 can observe a given star for a maximum of approximately 80 days, instead of the three-plus years of continuous staring enabled by Kepler. But in the case of HIP 41378, a happy coincidence enabled the detection of five different companions within the available period of data collection (Figure 2). Two of them have repeating transits and the other three have a single transit each.

Figure 2. Light curves for five transiting planet candidates around HIP 41378

Phase-folded light curves for each of the candidate planets in the HIP 41378 system. Adapted from Figure 2 in Vanderburg & al. 2016.

A few definitions are in order. “High-multiplicity” is now in general use as a descriptor for systems with three or more planets. “Mixed-mass” is (I believe) my own coinage, referring to systems with at least one low-mass planet (an object less massive than about 50 Earth masses [50 Mea] with a bulk composition dominated by heavy elements) and one gas giant planet (an object of 50 Mea or more with a bulk composition dominated by hydrogen/helium). “Rich mixed-mass” is my own concept, referring to mixed-mass systems containing a minimum of two low-mass planets. (According to my terminology, then, all “rich mixed-mass” systems are automatically “high-multiplicity.”)
Note that all three of these descriptors apply to our Solar System. To date, however, fewer than 20 exoplanetary systems – among the many thousands now known – can be described as “rich mixed-mass.” HIP 41378 adds to their number.
Among systems published to date, the one that most closely resembles HIP 41378 is our friend Kepler-90, which supports an astonishing ensemble of seven planets ranging in radius from 1.2 Rea (Earth-like) to 11.3 Rea (Jupiter-like) on orbits shorter than a single Earth year.
Before cueing the Hallelujah Chorus, however, we should be aware of the limits of the K2 data. As the investigators concede, only two of the candidate planets around HIP 41378 have been formally validated by analysis of at least two transits. These two are HIP 41378 b and c, with respective orbital periods of 15.6 days and 31.7 days. For the other three candidates, Vanderburg & colleagues used the duration of each recorded transit to distinguish among them. They applied the letters d through f to designate objects with increasingly longer transit times and (potentially) longer orbital periods.
The investigators then calculated the probability that one or more of the candidates with a single transit were false positive detections. Using the K2 dataset, they found that a system with five transiting planets was 100 million times more likely than a system with two planets and three false positives. But they also found that the five-planet model was only 200 times more likely than a model with four planets and one false positive. Thus, the five-planet interpretation is probable but hardly certain.
The SIMBAD database assigns a spectral type of G1 IV to the host star, HIP 41378. The investigators prefer to describe it as a “slightly evolved late F-type star.” Its mass is 1.15 times Solar (1.15 Msol),  similar to Kepler-90 at 1.13 Msol. Its metallicity (endowment of elements heavier than hydrogen) is -0.11, meaning that the star is less enriched in metals than our Sun. Its distance is estimated at 116 parsecs, much nearer than most systems in the Kepler sample. Nevertheless, HIP 41378 still lies outside the region where radial velocity searches have typically been able to identify low-mass planets in the Super Earth range.
Vanderburg & colleagues argue that candidates HIP 41378 d through f have likely orbital periods exceeding 100 days. Their best guess for each period appears in Table 1. For simplicity, both the table and Figure 1 depict the five planets in alphabetical order, since the confidence intervals for the period of planet d overlap with those for planet e. Luckily, the estimates for planetary radii are more firmly grounded in transit data.
Table 1. HIP 41378 system parameters
Tags: Radius = planet radius in Earth units (Rea); Preferred Period = likely orbital period in days; Period Range = likely range in days of orbital period.
According to comparisons with transiting planets that have well-constrained mass estimates, planets b and c have likely masses in the range of 2 to 10 Mea; planet d is in the range of 5 to 26 Mea; planet e is a tween in the range of 15 to 40 Mea; and planet f is a gas giant in the range of 63 Mea (the mass of HAT-P-18b) to 318 Mea (the mass of Jupiter). Given their ample radii, even the smallest of these planets must have hydrogen/helium envelopes.
Despite the absence of purely rocky planets, HIP 41378 counts as a rich mixed-mass system. Its assortment of radii likely corresponds to an assortment of planetary species and bulk compositions. Like Kepler-90, it resembles a compact planet sampler (Figure 3), offering a planet population almost as diverse as our own Solar System within a space comparable to the orbital radius of the Earth.
Vanderburg & colleagues note that the system’s host star is bright enough for high-precision radial velocity measurements. Such data could confirm the reality of the transiting candidates and constrain the mass and orbital period of HIP 41378 f, and potentially of the smaller planets as well. The information could also help us understand the formation process of systems like HIP 41378 and our own Solar System.

Figure 3. A chocolate candy sampler


Andrew Vanderburg, Juliette C. Becker, Martti H. Kristiansen, Allyson Bieryla, Dmitry A. Duev, Rebecca Jensen-Clem, Timothy D. Morton, David W. Latham, Fred C. Adams, Christoph Baranec, Perry Berlind, Michael L. Calkins, Gilbert A. Esquerdo, Shrinivas Kulkarni, Nicholas M. Law, Reed Riddle, Maissa Salama, Allan R. Schmitt. Five Planets Transiting a Ninth Magnitude Star. Astrophysical Journal Letters, in press. Abstract: 2016arXiv160608441V


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