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Rocky Planets Around Cool Stars

Final Report Summary - ROPACS (Rocky Planets Around Cool Stars)

Project context and objectives

The ROPACS Initial Training Network (December 2008 - November 2012) includes the University of Hertfordshire (United Kingdom (UK), coordinating), Institute of Astronomy (Cambridge, UK), IAC (Tenerife, Spain), Max-Planck institute (Munich, Germany), Centre for Astrobiology (Madrid, Spain), and the MA-Observatory (Kiev, Ukraine), as academic partner institutes. Europe's largest space-engineering company Astrium is the industrial partner (Stevenage HQ, UK), and there are additional associated partners in Leiden (Netherlands), Dublin (Ireland), Madrid (Spain), Paris (France), Lisbon (Portugal), and East-England (UK). The network includes approximately 70 research astronomers and engineers, and European Commission funding employed 15 Marie-Curie research fellows at the partner institutes (11 Early-Stage-Researchers studying for PhDs, and 4 Experienced Researchers doing post-doctoral research).

ROPACS focuses on searching for and studying exoplanets (planets around other stars), in particular looking towards 'rocky planets around cool stars'. The transit and radial velocity techniques are paramount in ROPACS science, and a wide range of major facilities have been exploited. On the ground, the WFCAM Transit Survey (WTS) using the UK Infrared Telescope is a centerpiece, with a wide range of world-leading telescopes and survey databases providing follow-up and complementary studies. In space, the CoRoT and Kepler observatories have been used, and work has been carried out to foster proposed Cosmic Vision missions including the Exoplanet Characterisation Observatory (EChO).

The main project objectives are:

1. develop exploitation tools to search the new WTS survey for transiting planets;
2. carry out follow-up campaigns (on a variety of telescopes) to refine the list of candidate transits;
3. develop radial velocity search techniques to improve extra-solar planet sensitivity;
4. use these techniques to search for new planets around cool/nearby stars;
5. Use / develop the most ambitious facilities for transit measurements;
6. measure primary/secondary eclipses - planets passing in-front/behind their host stars;
7. develop and use the latest theoretical models to study the properties of planets and their host stars;
8. carry out research and development on proposed European Space Agency Cosmic Vision missions for exoplanet science.

The work of the network

The ROPACS community has developed and optimised approaches to search the WTS for transiting planets. We have implemented large cross-network follow-up campaigns on large international telescopes to identify false positives and reveal new planetary systems. These systems have been studied using a range of methods to determine planet and star properties that have revealed exotic planets and multiple systems.

Existing radial velocity data has also been analysed, using new techniques that combine Bayesian analysis with various noise models and stability arguments. Additional activity analysis and careful assessment of periodic signals has led to the identification of many new planets and planetary systems that were beyond the reach of previous search methods.

The latest large-scale sky surveys have been used to reveal wide companions with planet-like temperatures. With temperatures down to about 500 K (and possibly as cool as about 300 K) these objects are providing the most direct means of studying emission spectra in this temperature range, and are being used to guide/improve theoretical understanding of such ultra-cool atmospheres.

Through a collaborative academia / industry programme ROPACS has been working to optimally combine science requirements and technology readiness. In this context space engineers have been working with the astronomical community to focus future studies onto the most exciting systems for which technology will be capable of providing the next level of exoplanet studies.

Main results achieved so far

The WTS has been used to identify two new transiting exoplanets. WTS-1b is an inflated hot Jupiter with four Jupiter masses and a radius 1.5 times that of Jupiter. This represents one of the largest radius anomalies amongst the known high mass (3-5 Mj) hot Jupiters, and could be explained by ohmic heating - where a surface wind across the planetary magnetic field interacts with deeper layers and acts like a battery. WTS-2b is also inflated (1.3 Rj) but has a lower mass of 1.1Mj. It orbits unusually close to its host star such that a WTS-2b year is just one day. This extreme planet is testing our understanding of how planets migrate due to tidal interaction with their host star.

New Bayesian methods have discovered numerous exciting new planets using the radial velocity technique. Habitable zone super-Earths have been discovered around HD40307 and also Tau Ceti (the closest single Sun-like star). Such potentially habitable worlds appear to be quite common, though are a major challenge to identify. Multiple planet systems are also becoming more common, such as HD10180 that has been shown to harbour nine planets (one more than our own solar system).

Cool star properties are a continuing challenge to understand, and mass-radius data and activity from WTS systems have provided important measurements at the low mass stellar extreme. WTS has also revealed some exotic binary systems, with ultra-short periods. Such systems cannot be described by commonly used theory, and must be explained by an improved understanding of binary evolution.

The overlap between the coolest stars and giant planets has also been studied, revealing extremely cool sub-stellar (brown dwarfs with < 7 % of a solar mass) companions to stars and stellar remnants. Such objects can be used as test-beds to improve our knowledge of ultra-cool atmospheres (that enshroud planets as well as brown dwarfs) and evolution, since such companions should have formed in the same location and at the same time as their host star.

Looking to the future, improved radial velocity techniques applied to even cooler (lower mass) stars could fully probe for Earth-mass planets via a new generation of near-infrared high resolution spectrographs. And through supporting science for the proposed EChO mission (and other observatories such as the James Webb telescope) we are pursuing a path that will produce optimised targets for exoplanet atmosphere studies. Our new bright M dwarf catalogue is contributing to this forward looking approach, and ongoing studies aim to point the way to the most exciting cool star planets for study in the coming years.

Expected final results (looking to the future)

As the number of transiting planets increases it will become possible to fully understand the most exotic objects, putting the whole population in its appropriate context. A broad understanding of how planets form and evolve is of great interest to society, as it helps define our place in the cosmos. Knowledge of Earth-mass planets and the habitable zone population allows us to consider life around other stars, and in general conduct astrobiology. This research generates high impact in society as well as in the scientific field.

A thorough understanding of exoplanet populations requires much complex theory, both of cool star interiors and atmospheres, and also of planetary atmospheres. By expanding the studies of M dwarfs (eclipsing and variable), we will improve our ability to determine host star sizes (and the sizes of planets that transit them). And by studying well constrained ultra-cool atmospheres, the physics of such environments can be explained - allowing us to de-code future observations of giant planets around nearby stars.

Ultimately, the study of habitable zone super-Earth atmospheres will provide the most ambitious next step in the exoplanets field, but future space-based missions are crucial and must be fostered from an early stage to gain highest impact.

Contact details

ROPACS coordinator:
Professor David Pinfield
Centre for Astrophysics Research
The University of Hertfordhsire
College Lane
Hatfield, AL10 9AB, UK

ROPACS website: http://star.herts.ac.uk/RoPACS