Extrasolar Planets and Brown Dwarfs


Dynamical modelling of radial velocity data in a search for exoplanets

Mikko Tuomi, Hugh Jones, David Pinfield

When searching for planets orbiting nearby stars in tightly packed orbital configurations, it is more often than not the case that the data are analysed independently of the dynamical integrations of the orbits. However, simultaneous modelling accounting for both information in the data and dynamical analyses can provide more robust results for such problems. This can enable the exclusion of dynamically unstable solutions from the set of orbital configurations supported by data enabling more trustworthy detections of dynamically packed exoplanet systems in the Solar neighbourhood. The project would use a large database of high quality radial velocities for stars in the solar neighbourhood and obtain additional measurements to follow-up the most interesting signals discovered with further radial velocities as well as with state of the art direct imaging systems.


Nearby low-mass planets around Solar type stars

Fabo Feng, Mikko Tuomi, Hugh Jones

The search for rocky earth-like planets in radial velocity data sets is hampered by stellar activity noise, even in some of the quietest stars that are routinely monitored for radial velocity variations. Although techniques aimed at searching for weak signals in noisy data are now able to detect multiple planet systems, their effectiveness can be augmented with accurate noise models. This project will make use of large archival data sets from precision radial velocity surveys by probing the signatures of stellar activity in spectral lines in an effort to provide improved noise models. This difficult problem will be calibrated by Gaia observations of known systems where their proximity and planet/star mass ratios give rise to relatively large astrometric signals and thus we will work with data from Gaia to make joint astrometric and radial velocity solutions.  This provides the opportunity to dramatically improve the robustness and sensitivity of searches for Earth-like planets orbiting nearby stars. This analysis can provide the calibration to make a uniform analysis of radial velocity data allowing for a reliable determination of the exoplanet mass function in the solar neighbourhood.


Exoplanet mass function for nearby stars

Hugh Jones, Mikko Tuomi

The advent of powerful sub-mm observatories means that dust disks are now being routinely discovered around many nearby stars. The combination of alignment information with large numbers of radial velocity measurements offers the opportunity to construct a sample of nearby stars for which the planetary mass can be precisely determined. Previous determinations of planetary mass by radial velocities suffer from an orbital inclination uncertainty and those from transits rely on a mass-radius relationship. This project thus  represents a direct method to determine the exoplanet mass function based on nearby stars. Much of the project will be concerned with quantifying the activity and geometric biases within the sample of nearby stars considered.


Discovering Y dwarfs from the WISE observatory

David Pinfield, Federico Marocco, Hugh Jones

We are conducting a search for the faintest Y dwarfs detectable by NASA's orbiting WISE telescope. Y dwarfs are brown dwarfs, star-like objects that cool and fade with time. With temperatures of 250-500K or lower, Y dwarfs share many characteristics with giant planets and the coolest known example has a temperature <0C (cooler than room temperature). WISE has scanned the whole sky multiple times in the mid-infrared where very cool brown dwarfs are brightest, and in 2011 discovered the first Y dwarfs. Our search method employs Bayesian inference at low signal-to-noise, and has the potential to yield cooler and/or closer objects with very high proper motion. The student will join the programme and take part in the process of followup and analysis leading to new discoveries. The student will also expand the search to lower galactic latitudes where discovery is more challenging, but hunting grounds are un-tapped. It is possible that WISE could reveal a Y dwarf within the distance of Proxima Centauri - currently the Sun's nearest neighbour.


The exoplanets brown dwarf connection - characterising the coolest companions

David Pinfield, Niall Deacon, Federico Marocco

Very cool companions are excellent test-beds that help astronomers understand the physics of ultracool atmospheres. They can form in a variety of ways - they may be brown dwarf companions that formed in situ, or giant planets that migrated out to large orbits. In this project the student will develop methods to identify such systems in the UKIDSS and fast-growing VISTA surveys. This will establish a sample companions for which the new GAIA telescope (currently taking data at L2) will provide superb measurements of the primary stars. By studying these ultracool companions and their primary stars the student will determine their physical properties (temperature, surface gravity, mass, age, composition). And in some cases detailed spectroscopy could provide a tool for differentiating between very low mass brown dwarfs and cool giant planets.


M dwarfs and their planetary systems

David Pinfield, Federico Marocco, Hugh Jones

Cool M dwarf stars are particularly compelling targets for extrasolar planet hunting, since their small size, low mass, and faint luminosities lead to enhanced signatures of extrasolar planet companions, and a greater capacity to study their physical and atmospheric properties. This opens up potential for studying the atmospheres of super-Earths with the next generation of space-based observatories such as the James Web Space Telescope and proposed missions like Ariel and Twinkle. Our group has recently published two major new M dwarf catalogues in which we aim to identify planets for future study with these missions. In this project the student will take part in the characterisation of several M dwarf subsamples, and contribute to the search for (i) faint ultracool companions (brown dwarfs and giant planets) that cause an infrared excess in their M dwarf hosts, (ii) young M dwarf planets that could be directly imaged with adaptive optics instruments, and (iii) close-in M dwarf planets that could tidally affect the spin and activity of their hosts. A range of observational methods will be used, building on our access to the new spectroscopic database from the Chinese LAMOST telescope, and near-future data from GAIA which will provide unprecedented accuracy for the study of M dwarfs and their orbital systems.


EXOhSPEC

Martin, Jones

This project is to build the so-called EXOhSPEC prototype spectrograph (Exoplanet high resolution spectrograph). It will be entirely built from catalogue components but will utilise technologies not deployed in astronomical spectrographs. EXOhSPEC will be tested on the Sun and local stars both in the laboratory, on local automated telescope as well as with telescopes in Thailand. The final version is destined for the Thai National Telescope. It is intended to have the fewest possible optical surfaces for a high resolution spectrograph and its efficiency and small size will make it highly attractive. The key development that we wish to introduce is active metrology of the system which enables us to envisage a small athermal design made from off the shelf parts which is replicable and scalable. The aim of the project is to be able to build a prototype to significantly extend the reach of precision radial velocities to higher precisions and efficiencies.