Since we’ve recently been discussing astrometry, the discipline that measures star distances and movements, now would be a good time to look at two significant projects that go beyond optical methods to use radio astrometry in planet hunting. The Radio Interferometric Planet Search (RIPL) will draw on the Very Long Baseline Array, ten dish antennae spanning more than 5000 miles, and the 100-meter Green Bank telescope in West Virginia. The target: 29 active low-mass stars to be examined in a three-year planet hunt.
The targets are significant because they’re a kind of star that’s currently out of reach for radial velocity techniques. All are M dwarfs that are active, meaning they display ‘starspots’ (analogous to sunspots), flares or other activity in their chromospheres. The more active a faint star like this, the more likely that radial velocity measurements will be distorted with a ‘jitter’ that disturbs the precision of the measurement. RIPL ought to be able to sort out the situation and bring planet hunting to this niche.
The new observing project ought to be useful, then, in fleshing out our slowly growing knowledge of M dwarf systems. The authors of a recent paper outlining the project go so far as to say this:
Radio astrometric searches can determine whether or not M dwarfs, the largest stellar constituent of the Galaxy, are surrounded by planetary systems as frequently as FGK stars and if the planet mass-period relation varies with stellar type. The population of gas giants at a few AU around low mass stars is an important discriminant between planet formation models.
We’ve seen that issue in play recently with the planet discovered around GJ 674, which seems to reinforce the core accretion model of planet formation. What RIPL brings to the table is the ability to find lower mass planets in long-period orbits, and to define precise astrometric positions for these stars and their planets. Interestingly, radio astrometry is quite useful in making measurements of distant objects, including the proper motion of some pulsars and even the motion of Sagittarius A*, the apparent black hole at the center of the Milky Way. So employing radio techniques in the planet hunt is complementary to existing searches.
Beyond RIPL, the proposed radio telescope called the Square Kilometer Array (SKA) would have a collecting area roughly 200 times that of the VLBA. Here we achieve the possibility of detecting Earth mass planets around such stars, although the instrument will be sensitive enough to extend the search from M dwarfs to stars much like the Sun. For this one, though, we wait. Construction is not planned for more than a decade.
The overview paper is Bower et al., “Radio Astrometric Detection and Characterization of Extra-Solar Planets: A White Paper Submitted to the NSF ExoPlanet Task Force,” available here.
Predicting the frequencies of diverse exo-planetary systems
Authors: J.S. Greaves, D.A. Fischer, M.C. Wyatt, C.A. Beichman, G. Bryden
(Submitted on 6 Apr 2007)
Abstract: Extrasolar planetary systems range from hot Jupiters out to icy comet belts more distant than Pluto. We explain this diversity in a model where the mass of solids in the primordial circumstellar disk dictates the outcome. The star retains measures of the initial heavy-element (metal) abundance that can be used to map solid masses onto outcomes, and the frequencies of all classes are correctly predicted. The differing dependences on metallicity for forming massive planets and low-mass cometary bodies are also explained. By extrapolation, around two-thirds of stars have enough solids to form Earth-like planets, and a high rate is supported by the first detections of low-mass exo-planets.
Comments:
5 pages, 2 figures; accepted by MNRAS
Subjects:
Astrophysics (astro-ph)
Cite as:
arXiv:0704.0873v1 [astro-ph]
Submission history
From: Jane Greaves [view email]
[v1] Fri, 6 Apr 2007 11:04:07 GMT (28kb)
http://arxiv.org/abs/0704.0873
It’s exciting that we have such a broad array of techniques available to search for ESP’s. I’m certain we’ll have a growing catalogue of rocky terrestrials in 15-20 years.
Hi Chris & Larry
As always, thanks for the newsbites Larry.
As for ESPs (cool acronym) I agree with you Chris. We have over 200 in our catalogue after just 12 years of discoveries, so the quantity and quality after 15-20 years of better ground searches and the space telescope/interferometer missions will be phenomenal.
But will we spot an Earth? Or will that have to wait for the Terrestrial Planet Imager, sometime after 2025?
The VLBA was used to measure the proper motion of a Galaxy(!).
Is RIPL doing wobble, proper motion, or something else?
The Allen array is further along than the SKA. It’s being built now.
The Dynamics-Based Approach to Studying Terrestrial Exoplanets
Authors: David Charbonneau, Drake Deming
(Submitted on 7 Jun 2007)
Abstract: One of the great quests of astronomy is to obtain the spectrum of a terrestrial planet orbiting within the habitable zone of its star, and the dominant challenge in doing so is to isolate the light of the planet from that of the star. Dynamics-based methods separate these signals temporally, whereas imaging techniques do so spatially. In light of the overwhelming dominance of dynamics-based methods over the past decade, we challenge the notion that spectra of terrestrial planets necessarily require extreme imaging methods. We advocate that some resources be committed to refining the proven technologies of radial-velocity measurements, transit photometry, and occultation spectroscopy (i.e. emergent infrared spectra obtained at secondary eclipse). We see a particularly attractive opportunity in M-dwarfs, for which the habitable zone is close to the star, increasing the probability and frequency of transits, and the amplitude of the induced radial-velocity variation. Such planets could be discovered by a dedicated ground-based transit survey of the 10,000 nearest M-dwarfs. The favorable planet-star contrast ratio would make these planets ideal targets for the study of their atmospheres with the technique of occultation spectroscopy.
Comments: submitted to the Exoplanet Task Force (AAAC), 2 April 2007
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0706.1047v1 [astro-ph]
Submission history
From: David Charbonneau [view email]
[v1] Thu, 7 Jun 2007 17:39:11 GMT (333kb)
http://arxiv.org/abs/0706.1047
Science With The Australian Square Kilometre Array Pathfinder
Authors: Simon Johnston (ATNF, CSIRO), et al
(Submitted on 14 Nov 2007)
Abstract: The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz.
The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science.
ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on Earth and one of two sites selected by the international community as a potential location for the SKA.
In this paper, we outline an ambitious science program for ASKAP, examining key science such as understanding the evolution, formation and population of galaxies including our own, understanding the magnetic Universe, revealing the transient radio sky and searching for gravitational waves.
Comments: Accepted for publication in PASA. 14 pages, 6 figures
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0711.2103v1 [astro-ph]
Submission history
From: Simon Johnston [view email]
[v1] Wed, 14 Nov 2007 02:50:33 GMT (617kb)
http://arxiv.org/abs/0711.2103
The Search for Stellar Companions to Exoplanet Host Stars Using the CHARA Array
Authors: Ellyn K. Baines, Harold A. McAlister, Theo A. ten Brummelaar, Nils H. Turner, Judit Sturmann, Laszlo Sturmann, Stephen T. Ridgway
(Submitted on 28 Mar 2008)
Abstract: Most exoplanets have been discovered via radial velocity studies, which are inherently insensitive to orbital inclination. Interferometric observations will show evidence of a stellar companion if it sufficiently bright, regardless of the inclination.
Using the CHARA Array, we observed 22 exoplanet host stars to search for stellar companions in low-inclination orbits that may be masquerading as planetary systems. While no definitive stellar companions were discovered, it was possible to rule out certain secondary spectral types for each exoplanet system observed by studying the errors in the diameter fit to calibrated visibilities and by searching for separated fringe packets.
Comments: 26 pages, 5 tables, 8 figures
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0803.4131v1 [astro-ph]
Submission history
From: Ellyn Baines [view email]
[v1] Fri, 28 Mar 2008 14:32:00 GMT (108kb)
http://arxiv.org/abs/0803.4131