[X] My Assistant

[Tom Womack]

http://news.bbc.co.uk/1/hi/sci/tech/6611557.stm

is reporting that Corot has found its first planet. I can't find an arxiv paper about this, or even a press release, but there are many here better at squirreling out data releases than me.

1.3Mj, 1.8Rj so it's a very inflated planet, 1.5-day orbit around a 'star quite similar to the Sun' might account for that. In the Monoceros field (Corot is now pointing at the Scutum/Aquila field).

[Jyril]

Well, the negative side of the COROT survey is that the search is limited to closely orbiting transiting planets so we get a very biased sample. Which is far better than nothing, of course.

A microlensing planet survey equipped even with a relatively small telescope could find Earth-mass or smaller planets in any orbital distance (including free-floating terrestrial planets)! It could detect every planet of the Solar System except Mercury, which is not massive enough and orbits too close the Sun. The obvious downside is of course that the lensing events are unique and no physical properties of the planets can be studied. But it could give a good sample of planets around very different kinds of stars.

[JRehling]

Well, the negative side of the COROT survey is that the search is limited to closely orbiting transiting planets so we get a very biased sample. Which is far better than nothing, of course.

Check my math in trying to characterize the bias.

Given two similar planets orbiting two similar stars, but with one planet N times farther from its star than the other, the ratio of likelihood of detection in a short time frame should be N^2.5. That is, the probability of appropriate geometry for a transit is decreased by N for the farther planet, whereas the probability of a transit taking place at the right time is a function of the orbital period, which introduces another factor of N^1.5.

For example, if Earth were orbiting at 5 AU, it would be precisely 1/5 as likely for its orbit to transit the Sun as seen from afar, and if it did, it would do so about 1/11th as often. So a factor of 5 in distance translates to a factor of 55 in transit observations. A factor of 10 in distance translates to a factor of 300 in transit observations.

The temporal factor is mitigated as the observations continue. Given a mission lasting Y years, we'd get one observation of every transiting planet with a period <=Y, two observations of every transiting planet with a period <=Y/2, and a probability Y/X of one observation of every transiting planet with a period X longer than Y.

The diameter of the planet is also a minor factor. Jupiter might graze the Sun's disk whereas a Pluto in the same location would just miss. As the planets get much smaller than the star, this factor almost vanishes.

COROT will survey a few different areas, none for more than 150 days or so, so repeat detections will be strictly limited to planets in close-in orbits. Single detections of planets farther out will (presumably!) take place, and could help us get an idea of the distribution of planets in different-sized orbits. But at some point out there, the data will be too sparse to make predictions significant.

So overall, I think it's going to be pretty sparing in telling us about the raw numbers of Venuses, Earths, and Neptunes. But a few data points would be a lot nicer than none.

[tacitus]

COROT will survey a few different areas, none for more than 150 days or so, so repeat detections will be strictly limited to planets in close-in orbits. Single detections of planets farther out will (presumably!) take place, and could help us get an idea of the distribution of planets in different-sized orbits. But at some point out there, the data will be too sparse to make predictions significant.

While I understand that there are two target regions between which COROT will alternate every six months, I hadn't realized they were planning to observe new locations within those region each time the spacecraft came back to them. I can imagine that at some point (maybe in an extended mission) they might decide to return to a spot previously observed in an attempt to observe repeat events caused by planets with longer orbits.

I guess it all depends on what sort of data they observe in the first few observational runs. If they find a number of great Earth-sized candidates in one region, I should have thought they would be tempted to return to it the next year.