Exoplanet surveys recently became precise enough to detect more than a single planet orbiting distant stars. We still observe these objects almost by chance, i.e. by looking for optical transit signals, which are generated when planets pass in front of their host stars.
The future space observatories and space missions like TESS promise to increase space observation capabilities much further. Such prospective technological advance fuels the curiosity of astronomers in advance, who strive to learn more about characteristics of planetary systems other than our own Solar System using readily available scientific equipment and appropriate data processing methods.
One example of such work is the draft version of paper published today on arXiv.org, where authors present a statistical analysis of M dwarf class stars from Kepler mission with aim to determine number of systems hosting more than a single planet.
In essence, the notion of such studies is not entirely new, because similar works have been published previously. However, it is interesting to note that every subsequent study had access to larger data sets and therefore yielded more plausible results. For example, this year one study (Fabrycky et al., 2014) reported that approximately 20 % of the planet host stars observed by single Kepler mission (Batalha et al., 2013) host at least one transiting planet, and among them – the first exoplanetary system found to have more than one transiting planet Kepler-9, discovered using transit method.
Other detection tools apart from those that are carried on Kepler space telescope have uncovered even wider array of exoplanets and even multi-planet systems and thus provided a basis for deeper analysis using methods of statistics. Multiple teams of astronomers have devised statistical models that were best-suited to reproduce the Kepler multi-planet yield, and now according to some estimates up to 80 % of planetary systems may host more than one exoplanet, while 85 % of planetary orbits in multiple-planet systems are inclined by less than 3° in respect to one another (Fang & Margot, 2012). Such data represents a quite obvious trend of co-planar planetary systems, scientists say.
Sarah Ballard and John Asher Johnson, the authors of the current study, based their work on some previous statements that the number of exoplanets estimated from the Kepler data may be underpredicted by as much as a factor of two (Lissauer et al., 2011; Hansen & Murray, 2013). The team decided to investigate how many planets are required per star, and within what mutual inclination range, to specifically recover the Kepler’s multi-planet yield around the least massive (and most abundant at the same time) stars, the M dwarfs. The authors note that the availability of the data and some inherent properties like the amplitude of transit signal were one of the main motives to select this class of stars for their study.
For their investigation, the team selected a sample of star hosts, or KOIs, from the publicly available NASA Exoplanet Science Institute (NExScl) database. The eclipsing binaries, blended stars and false positives determined by previous studies were eliminated prior to the data analysis. The final sample comprised 167 individual “objects of interest” orbiting 106 stars. Of them, 71 host one planet, 17 host two, 12 host three, 4 host four and 2 host five potentially identified exoplanets.
Statistical analysis of the data produced quite interesting results. According to the scientists, the multiplicity statistics of the Kepler exoplanet sample orbiting M dwarfs cannot be reproduced by assuming a single planetary system architecture. However, statistics of the multi-planet observations (i.e. hosting 2 or more transiting planets) available from selected sample of host stars could be quite accurately approximated using well-populated and approximately coplanar architecture of planetary systems.
The authors also note that the results of the study revealed three stellar properties which with, as they say, ‘modest significance’ and 95 % statistical confidence, can be used as predictors in order to determine whether a given M dwarf host a single planet, or multiple planets. These are: stellar rotation, height from galactic mid-plane, and metallicity. Systems with observed multiple transits are on average rotating more quickly, closer to the galactic midplane, and metal poor (i.e. latter indication of relatively old stars).
“Though individually the properties of the stellar hosts to multiple transiting planets are only modestly different from those hosting a single transiting planet, the potential implications are intriquing”, S. Ballard and J. A. Johnson note. They also mention the study by Dawson & Murray-Clay (2013), where authors posit that metal-rich stars host dynamically “hotter” planets on average, and this means that metal-poor stars should host “flatter” coplanar planetary systems, the assumption that supports the results of the current work.
The team say that more exoplanetary systems are needed to be included in the available data to robustly test the reliability of stellar parameters as predictors for architecture of M dwarf planetary systems.
Written by Alius Noreika