TESS: All Sky Survey for Transiting Planets

by Paul Gilster on March 26, 2008

I’ve never met George Ricker, but in at least one respect I believe he thinks the way I do. Ricker is senior research scientist at MIT’s Kavli Institute for Astrophysics and Space Research, and he’s someone who can connect the exoplanetary systems we study with places we might eventually go. As witness this comment in a discussion of a planned satellite-based observatory being designed at the Institute:

“Decades, or even centuries after the TESS survey is completed, the new planetary systems it discovers will continue to be studied because they are both nearby and bright. In fact, when starships transporting colonists first depart the solar system, they may well be headed toward a TESS-discovered planet as their new home.”

It’s wonderful to see a ‘when’ rather than an ‘if’ when referring to starships, even though everyone concerned can appreciate the blue-sky nature of the comment. For my part, I’ll take whatever the physics will bear, from close-up imagery of terrestrial exoplanets to robotic probes pushed by fusion or lightsails, and just maybe down the line, vessels carrying human crews. It’s a goal worth dreaming about.

Transiting 'hot Jupiter'

TESS refers to the Transiting Exoplanet Survey Satellite, which has caught a bit more media buzz than might be expected because Google is among its sponsors. Remember when we thought of Google as a search engine? Now it’s all over the place, mapping not only the planet but also the heavens through Google Sky. The synergy between the data collection capabilities of wide-field digital cameras and a company that makes its money sifting through information is obvious. TESS will use six such cameras, covering the sky in its entirety and measuring the brightness of some two million stars in total.

Image: Artist’s conception of a transiting ‘hot Jupiter.’ An all-sky survey like TESS should track down more of these, with the potential for landing smaller terrestrial-class worlds as well. Image copyright Mark A. Garlick / markgarlick.com.

The targets: G, K and M-class stars. Make the assumption that one star in a thousand makes a transit as seen from Earth. Observe two million stars and you’re looking at a couple of thousand transiting worlds, and perhaps more depending on how accurate our emerging views of planetary formation are. Design work for TESS is scheduled to be completed this year (you can read more about that and the scientific partnerships MIT has entered into on this project in this news release). For more specifics, David Latham (Harvard-Smithsonian Center for Astrophysics) offered a helpful description of TESS as well as the Kepler mission in a presentation he gave at the California Institute of Technology in 2007. The CfA is partnering with MIT on the TESS project.

Will TESS-discovered planets form the target list for our first interstellar probes? Conceivably, although that assumes that funding to build and launch the satellite is forthcoming. The earliest launch date is 2012. My own hunch is that within that time we may well have identified an Earth-sized planet that is unqualifiedly in the habitable zone of an M-dwarf, one whose orbital position is far less ambiguous than either Gliese 581 c or d. For that matter, if Greg Laughlin’s team can work out the arrangements, we may even have good radial velocity data on rocky worlds around Centauri B. So I wouldn’t put too much emphasis on TESS being first, but the more eyes we have for the exoplanet hunt the better.

yeti March 26, 2008 at 11:49

yeah i agree the more exoplanet missions the better :) but by 2012 kepler should have some candidates. It launches 2009 for a 3.5 year mission. Giving 3 transits of rocky worlds in the HZ of a G type star in just over 3 years.

although if data analyses is as slow on kepler as it is on CoRoT we will be waiting a while beyond 2012.

Hans Bausewein March 26, 2008 at 16:43

Synergy between Googles business activities and transiting planet searching could be good for both. Maybe Google wants to improve its image searching.

I hope that enough of the process becomes available for others to review and build on.

andy March 26, 2008 at 18:14

As transit surveys push out to longer-period planets, we may start to see detections of moons or ring systems. I wonder if anyone would risk a guess as to when the first exomoon will be discovered…

David March 27, 2008 at 0:20

Andy,

My hunch is that it will be some years before an exomoon is discovered because while the probability of a transit detection increases linearly with decreasing semi-major axis, the Hill radius, the maximum radius that a planet can hold on to a moon, both decreases linearly and the maximum mass of such a moon also decreases. In other words, while close-in exoplanets are more likely to be detected by transits, any moon will be both small and close to the planet. Also the larger the moon, the easier it can be detected.

Massive moons, say Earth-size, are only possible around giant planets with periods of greater than ~1 year around solar-type stars and here the probability of a transit is typically less than a few percent. So even if you are lucky to detect a candidate, you would still have to wait another year or more for confirmation. I think it likely that any discovery of an exomoom by transit will likely be done by a telescope in space.

dad2059 March 27, 2008 at 8:37

A discovery of an Earth-type planet or the discovery of another biological habitat within our own Solar System could be a needed shot in the arm for the human space program. Governments and/or private companies would have motivation to develop outside of the box propulsion and AI technologies to further explore these new realms.

philw1776 March 27, 2008 at 11:50

My bet is an exomoon discovery in 2010 by Kepler, assuming the launch and orbit transit is successful. A big fat jovian close in around an M star with satellite companions.

andy March 28, 2008 at 19:03

David: you may be being overly pessimistic about the possibilities of Earthlike moons: theoretical studies suggest that at least for planets on circular orbits, an Earth-mass moon could remain stable around planets fairly close to the star: for example, Rho Coronae Borealis b (a=0.22 AU, P=40 days) could conceivably host an Earth-mass satellite. (See this paper for details)

David March 29, 2008 at 1:05

Andy,
Thanks for the reference. my thinking was based on reading some time ago this paper:
http://adsabs.harvard.edu/abs/2002ApJ…575.1087B though looking at it again
I see that Barnes and O’Brien mention the interesting notion that an exomoon transit detection might be easiest if the planet is orbiting an M dwarf.

ljk May 4, 2008 at 23:31

Analytic Approximations for Transit Light Curve Observables, Uncertainties, and Covariances

Authors: Joshua A. Carter, Jennifer C. Yee, Jason Eastman, B. Scott Gaudi, Joshua N. Winn

(Submitted on 2 May 2008)

Abstract: The light curve of an exoplanetary transit can be used to estimate the planetary radius and other parameters of interest. Because accurate parameter estimation is a non-analytic and computationally intensive problem, it is often useful to have analytic approximations for the parameters as well as their uncertainties and covariances.

Here we give such formulas, for the case of an exoplanet transiting a star with a uniform brightness distribution. We also assess the advantages of some relatively uncorrelated parameter sets for fitting actual data. When limb darkening is significant, our parameter sets are still useful, although our analytic formulas underpredict the covariances and uncertainties.

Comments: 33 pages, 14 figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.0238v1 [astro-ph]

Submission history

From: Joshua Carter [view email]

[v1] Fri, 2 May 2008 15:08:05 GMT (489kb)

http://arxiv.org/abs/0805.0238

ljk July 1, 2008 at 11:48

The NStED Stellar and Exoplanet Hosting Star Service

Authors: S. Ramirez, B. Ali, R. Baker, G.B. Berriman, K. von Braun, N-M. Chiu, D.R. Ciardi, J. Good, S.R. Kane, A.C. Laity, D.L. McElroy, S. Monkewitz, A.N. Payne, M. Schmitz, J.R. Stauffer, P.L. Wyatt, A. Zhang

(Submitted on 27 Jun 2008)

Abstract: The NASA Star and Exoplanet Database (NStED) is a general purpose stellar archive with the aim of providing support for NASA’s planet finding and characterization goals, stellar astrophysics, and the planning of NASA and other space missions.

There are two principal components of NStED: a database of (currently) 140,000 nearby stars and exoplanet-hosting stars, and an archive dedicated to high precision photometric surveys for transiting exoplanets.

We present a summary of the NStED stellar database, functionality, tools, and user interface. NStED currently serves the following kinds of data for 140,000 stars (where available): coordinates, multiplicity, proper motion, parallax, spectral type, multiband photometry, radial velocity, metallicity, chromospheric and coronal activity index, and rotation velocity/period.

Furthermore, the following derived quantities are given wherever possible: distance, effective temperature, mass, radius, luminosity, space motions, and physical/angular dimensions of habitable zone. Queries to NStED can be made using constraints on any combination of the above parameters.

In addition, NStED provides tools to derive specific inferred quantities for the stars in the database, cross-referenced with available extra-solar planetary data for those host stars. NStED can be accessed at this http URL

Comments: To appear in the Proceedings of the 253rd IAU Symposium: “Transiting Planets”, May 2008, Cambridge, MA. 4 pages, 4 figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0806.4611v1 [astro-ph]

Submission history

From: Stephen Kane [view email]

[v1] Fri, 27 Jun 2008 20:32:05 GMT (990kb)

http://arxiv.org/abs/0806.4611

ljk July 1, 2008 at 13:32

Toward a homogeneous set of transiting planet parameters

Authors: Guillermo Torres (CfA), Joshua N. Winn (MIT), Matthew J. Holman (CfA)

(Submitted on 26 Jun 2008)

Abstract: With 40 or more transiting exoplanets now known, the time is ripe to seek patterns and correlations among their observed properties, which may give important insights into planet formation, structure, and evolution.

This task is made difficult by the widely different methodologies that have been applied to measure their properties in individual cases. Furthermore, in many systems our knowledge of the planet properties is limited by the knowledge of the properties of the parent stars.

To address these difficulties we have undertaken the first comprehensive analysis of the data for 23 transiting planets using a uniform methodology. We revisit several of the recently proposed correlations, and find new ones involving the metallicity of the parent stars.

Comments: 4 pages including figures. To appear in Proceedings of IAU Symposium 253, “Transiting Planets”, May 2008, Cambridge, MA

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0806.4353v1 [astro-ph]

Submission history

From: Guillermo Torres [view email]

[v1] Thu, 26 Jun 2008 16:03:11 GMT (404kb)

http://arxiv.org/abs/0806.4353

ljk July 3, 2008 at 16:31

Predicting the Yields of Photometric Surveys for Transiting Planets

Authors: Thomas G. Beatty

(Submitted on 1 Jul 2008)

Abstract: Observing extrasolar planetary transits is one of the only ways that we may infer the masses and radii of planets outside the Solar System. As such, the detections made by photometric transit surveys are one of the only foreseeable ways that the areas of planetary interiors, system dynamics, migration, and formation will acquire more data.

Predicting the yields of these surveys therefore serves as a useful statistical tool. Predictions allows us to check the efficiency of transit surveys (“are we detecting all that we should?”) and to test our understanding of the relevant astrophysics (“what parameters affect predictions?”). Furthermore, just the raw numbers of how many planets will be detected by a survey can be interesting in its own right.

Here, we look at two different approaches to modeling predictions (forward and backward), and examine three different transit surveys (TrES, XO, and Kepler). In all cases, making predictions provides valuable insight into both extrasolar planets and the surveys themselves, but this must be tempered by an appreciation of the uncertainties in the statistical cut-offs used by the transit surveys.

Comments: To appear in the Proceedings of the 253rd IAU Symposium: “Transiting Planets”, May 2008, Cambridge, MA. 7 pages, 2 figures, 2 tables

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0807.0250v1 [astro-ph]

Submission history

From: Thomas Beatty [view email]

[v1] Tue, 1 Jul 2008 22:35:50 GMT (29kb)

http://arxiv.org/abs/0807.0250

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