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Terrestrial Worlds: The Devil in the Details

In a world dominated by short-term thinking, we tend to be driven by media cycles. That makes the coverage of science, among other subjects, problematic. Science operates through the analysis of detail as various minds subject a problem to hypotheses that can be tested experimentally. In other words, good science often takes time, which is why situations like the Gliese 581c story can be so frustrating.

Announce a habitable planet around another star and the media love you. Spend months and (if needed) years subjecting the habitability question to analysis and you’re not on the public radar. Many scientists have come to question whether Gliese 581c is remotely habitable; some even argue for habitability for the next planet out, Gliese 581d. We’re still trying to weigh the data, and such deliberate processes aren’t the sort of thing to replace the latest Hollywood starlet scandals on CNN.

The good scientist ignores media vagaries and proceeds with the painstaking details. The hunt for better tools for analysis is unceasing. Take the so-called Rossiter effect. It shows up in radial velocity data during a planetary transit, causing a distortion that seems to indicate a radial velocity shift. But there is no change in the velocity of the star being studied — the Rossiter effect only mimics such a change. The transiting planet has, in other words, meddled with the light being studied.

William F. Welsh and Jerome Orosz (San Diego State University) want to know whether this radial velocity ‘anomaly’ can be used to detect an Earth-like planet. After all, they point out, in the case of a terrestrial body orbiting beyond 0.5 AU, the amplitude of the Rossiter effect can be larger than the radial velocity of the host star’s orbit. The two astronomers simulated Rossiter effect signals and threw in observational noise to study the question. Could a useful signal be extracted from the noise of periodic star pulsations and other stellar variables?

The results aren’t encouraging for planets of Earth’s radius. But it does appear that the Rossiter effect could snare a slightly larger planet, especially as we move up to two Earth radii. Here’s the gist:

While using the Rossiter effect for discovering an Earth-like planet is certainly not competitive with the photometric technique (due to the vast multiplexing advantage wide-field photometry has), the Rossiter effect can used to provide an important confirmation of potential warm Earth-like planets. Since many of the most interesting transits that will be seen by the Kepler Mission will also be the most challenging, an independent confirmation of the transit via spectroscopy of the Rossiter effect may prove to be very helpful.

So what we’re dealing with here is a technique that can help us confirm the detection of a planet found by other methods. As Gliese 581c demonstrates, the more tools at our disposal to make such confirmations, the sooner we’ll be able to reach definitive conclusions. Such painstaking science rarely makes the headlines, but papers like this wind up leading to tools for scientists working at levels of detail that were once unimaginable. Further work will demonstrate the Rossiter effect’s viability, but looking for ways to extend our planet-hunting reach will doubtless yield the methods we’ll need to better understand what Kepler gives us.

The paper is Welsh and Orosz, “On Using the Rossiter Effect to Detect Terrestrial Planets,” Transiting Extrasolar Planets Workshop, ASP Conf. Ser. Vol 366, 2007; Eds. A. Afonso, D. Weldrake and Th. Henning, p. 176 (abstract).

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  • ljk July 31, 2007, 17:15

    Expansion of the Planet Detection Channels in Next-Generation Microlensing Surveys

    Authors: Cheongho Han (Chungbuk Natl. Univ., Korea)

    (Submitted on 28 Jul 2007)

    Abstract: We classify various types of planetary lensing signals and the channels of detecting them. We estimate the relative frequencies of planet detections through the individual channels with special emphasis on the new channels to be additionally provided by future lensing experiments that will survey wide fields continuously at high cadence by using very large-format imaging cameras.

    From this investigation, we find that the fraction of wide-separation planets that would be discovered through the new channels of detecting planetary signals as independent and repeating events would be substantial. We estimate that the fraction of planets detectable through the new channels would comprise ~15 — 30% of all planets depending on the models of the planetary separation distribution and mass ratios of planets.

    Considering that a significant fraction of planets might exist in the form of free-floating planets, the frequency of planets to be detected through the new channel would be even higher. With the expansion of the channels of detecting planet, future lensing surveys will greatly expand the range of planets to be probed.

    Comments: 6 pages, 3 figures, one table

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.4211v1 [astro-ph]

    Submission history

    From: Cheongho Han [view email]

    [v1] Sat, 28 Jul 2007 04:58:24 GMT (74kb)


  • ljk August 3, 2007, 16:04

    Effects of Orbital Eccentricity on Extrasolar Planet Transit Detection and Lightcurves

    Authors: Jason W. Barnes

    (Submitted on 1 Aug 2007)

    Abstract: It is shown herein that planets with eccentric orbits are more likely to transit than circularly orbiting planets with the same semimajor axis by a factor of (1-e^2)^{-1}. If the orbital parameters of discovered transiting planets are known, as from follow-up radial velocity observations, then the transit-detected planet population is easily debiased of this effect. The duration of a planet’s transit depends upon of its eccentricity and longitude of periastron; transits near periastron are shorter, and those near apoastron last longer, for a given impact parameter. If fitting for the stellar radius with the other transit parameters, this effect causes a systematic error in the resulting measurements. If the stellar radius is instead held fixed at a value measured independently, then it is possible to place a lower limit on the planet’s eccentricity using photometry alone. Orbital accelerations cause a difference in the planet’s ingress and egress durations that lead to an asymmetry in the transit lightcurve that could be used along with the transit velocity measurement to uniquely measure the planet’s eccentricity and longitude of periapsis. However, the effect is too small to be measured with current technology.

    The habitability of transiting terrestrial planets found by Kepler depends on those planets’ orbital eccentricities. While Kepler will be able to place lower limits on those planets’ orbital eccentricity, the actual value for any given planet will likely remain unknown.

    Comments: 8 pages, 6 figures, to appear in PASP 2007 September

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0708.0243v1 [astro-ph]

    Submission history

    From: Jason Barnes [view email]

    [v1] Wed, 1 Aug 2007 23:30:53 GMT (96kb)