Tag Archives: HARPS

Sep 5 2012

Exoplanet Update

It’s been a busy couple of weeks for exoplanetary discoveries, but also for me, which explains why I’ve taken so long getting round to writing about them.

On the 28th of August, the Kepler mission announced the discovery of a unique binary star two planet system. The Kepler 47 family consists of a binary pair, a G-type star – about 84% as massive as the Sun, and a smaller M-type red dwarf roughly 36% of the Sun’s mass, but only 1.4% as luminous. Two planets have been observed to be orbiting the pair. The closest is of these is Kepler 47 (AB) b, estimated (from mass-radius relationships) to be between 7 and 10 Earth masses, but the error on this figure remains large. The outermost planet, Kepler 47 (AB) c, is Neptune-sized (16 – 23 Earth masses) and is orbiting within the habitable zone, although due to its large mass it is unlikely to fulfil the traditional requirements for planetary habitability. The configuration of the Kepler 47 system illustrates the fact that stable multi-planetary orbits can exist around binary stars, and brings the total of circumbinary planets to six.

Artist’s impression of the Kepler 47 system. (NASA/JPL-Caltech/T. Pyle)

On the 29th of August, a new planet was added to the Habitable Exoplanets Catalog (HEC) bringing the total to six (including: Gliese 581d and g, Gliese 677Cc HD 85512b, Kepler 22b). Super-Earth Gliese 163c was established to be orbiting within the habitable zone of its 0.40 Solar mass star by an  international team working at the European HARPS project. It completes an orbit in 26 days and has a mass no less than 6.9 times that of the Earth. The custodians of the HEC database have given Gliese 163c an Earth Similarity Index (ESI) rating of 0.73, establishing it as the 5th ‘most habitable’ exoplanet discovered to date, despite exhibiting possible surface temperatures of 60 °C or above.

Gliese 163 c infographic: Warm Superterran Exoplanet in the Constellation Dorado (PHL @ Arecibo/HEC)

Speaking to online science network io9, HEC lead scientist Professor Abel Méndez in the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo said, “Gliese 163c ranks fifth in our current list of six potentially habitable exoplanets because it is nearly twice the size of Earth and its temperature is also higher, but it’s still an object of interest for the search of biosignatures by future observatories.” The HEC has yet to assess Kepler 43 (AB) c, but it is not likely to fare well in habitability assessments due to its large mass.

Bringing my own (as-of-yet-unpublished, but in preparation) research into planetary habitable periods to the table, Kepler 43 (AB) c has a residence time within the habitable zone of approximately 3.9 billion years, whilst Gliese 163c can be expected to within the habitable zone for at least 22.6 billion years. The habitable zone is now populated by 8 planets (including the Earth), and looks a bit like this:

The habitable zone and confirmed habitable zone exoplanets. The dashed lines indicate differing models of cloud cover. Data points are not to scale. (Author’s own research)

It’s certainly an exciting time to be working in this field; nearly each new week brings another interesting discovery. Keep looking up!

 

Apr 12 2012

A Multiplicity of Worlds

This article was originally posted at the European Association of Geochemistry blog (click for link)

Undoubtedly the most exciting exoplanet news of the past week is the discovery of a star system with a total of 9 potential planets, surpassing even our own Solar System in terms of planetary diversity. University of Hertfordshire astronomer Mikko Tuomi discovered the bustling planetopolis around the enigmatic star HD 10180, a Sun-like G-type main sequence star 127 light years distant, using a probabilistic Bayesian analysis technique.

View of the sky around the star HD 10180 (center) Credit: ESO

HD 10180 has been known as a multi-planet system since 2010, but the last analysis of the HARPS data available for the star, carried out by Christopher Lovis last year, seemed to indicate a 6 or 7-planet system was most likely. However, the novel probabilistic methods used by Tuomi are more computationally intense than those previously applied, and confirm the findings of Lovis whilst also adding a further two planets to the planetary inventory of HD 10180.

Tuomi’s Bayesian method, which seeks to evaluate a number of possible scenarios to determine which is most consistent with the observations, finds that an orbital configuration including an eighth and ninth planet, with masses 5.1 and 1.9 times that of the Earth respectively, returns a 99.7% probability.

The planets themselves, denoted HD 10180 b through h, are a diverse bunch, including two Earth-mass terran planets, one superterran, five neptunian and one jovian-sized planet, and all are contained within 3.5 AU – roughly the distance of the asteroid belt between Mars and Jupiter in our Solar System. Despite their proximity, the orbits are predicted to be stable over astronomical time.

Orbital and size visualisation of the HD 10180 system, courtesy of Abel Mendez at the Planetary Habitability Laboratory. The blue-green area denotes the habitable zone. (click for more detail).

The image above, from the Habitable Exoplanets Catalog, provides a visualisation of the orbital system and a comparison of the sizes of the planets. Note that one neptunian, HD 10180 g, is within the habitable zone but is unlikely to be habitable given its large mass, at least not by our definition.

That’s an extraordinary array of sizes and shapes crammed into a comparatively small area, and unseats our Solar System, with a certain 8 planets (excluding trans-neputunian objects, asteroids and dwarf planets – sorry Pluto fans!), from atop the pile of planetary richness, all the while adding to our understanding of the mechanisms of planetary system formation.

Whilst this is certainly an exciting discovery, should we be surprised by the apparent ubiquity of multi-planetary systems? It would be more unusual if this architecture wasn’t the norm, given model predictions. Writing for his Scientific American blog Life, Unbounded, astrobiologist Caleb Scharf notes that the combined masses of the HD 10180 planets would only amount to roughly half that of Jupiter, and given the star’s similarity to our own Sun, its proto-planetary circumstellar disk should have contained a similar amount of material. Therefore, it wouldn’t be surprising if more planets lurked in the HD 10180 system somewhere!

In fact, the same could be said for any of the planetary systems we have detected so far as well as those that we find in the future. Our detection techniques remain biased towards massive, short-period planets that produce readily identifiable signals, particularly when using the radial velocity method, and we suffer from the fact that we have only been collecting data for a few years and so may have missed more orbitally distant, longer period planets.

However, as with most exoplanet discoveries, the detection of this diverse family of worlds serves to put our planet  into some wider perspective – to challenge the notion that Earth and this solar system are particularly unique, at least in an astronomical sense.

Solar systems, it seems, are everywhere.