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first quarter numbers

April 2nd, 2008

Back in 2002, Keith Horne gave a talk at the Frontiers in Research on Extrasolar Planets meeting at the Carnegie Institute in Washington and showed an interesting table:

At that time, there were more than two dozen active searches for transiting extrasolar planets, but only a single transiting planet — HD 209458 b — had been detected. Transits were generating a lot of excitement, but paradoxically, the community was well into its third straight year with no transit detections. The photometric surveys seemed to be just on the verge of really opening the floodgates, with a total theoretical capacity to discover ~200 planets per month.

It’s been six years, and the total transiting planet count is nowhere near 14,000. Most of the surveys on the table have had a tougher-than-expected time with detections because of the large number of false positives, and because of the need to obtain high-precision radial velocities on large telescopes to confirm candidate transiting planets. Indeed, the surveys that were sensitive to dimmer stars have largely faded out. It’s just too expensive to get high-precision velocities for V>15 stars. With the exception of the OGLE survey (which had been set up to look for microlensing during the 1990s, and which had established a robust pipeline early on) none of the surveys that employed telescopes with apertures larger than 12 cm have been successful. The currently productive photometric projects: TrES, XO, HATnet, and SuperWASP all rely on telescopes of 10 to 11 cm aperture to monitor tens of thousands to hundreds of thousands of stars, and all are sensitive to planets transiting stars in the V~10 to V~12 magnitude range. This magnitude range is the sweet spot: there are plenty of stars (and hence plenty of transits) and the stars are bright enough for reasonably efficient radial velocity confirmation.

Yesterday, SuperWASP rolled out 10 new transits at once, dramatic evidence of the trend toward planetary commoditization and of the fact that it’s getting tougher to make a living out on the discovery side. The detection of new planets is growing routine enough that in order to generate a news splash, you need multiple planets, and the more the better. This inflationary situation for new transit news is highly reminiscent of where the Doppler surveys were at seven years ago. For example, on April 4, 2001, the Geneva team put out a press release announcing the discovery of eleven new planets (including current oklo fave HD 80606b).

I’d like to register some annoyance with this latest SuperWASP announcement. There are no coordinates for the new planets, making it impossible to confirm the transits. There is no refereed paper. The data on the website are inconsistent, making it hard to know what’s actually getting announced. I was astonished, for example, that WASP-6 is reported on the website to have a radius 50% that of Jupiter, and a mass of 1.3 Jovian masses:

That’s nuts! If the planet is so small, why is the transit so deep? And a 2200 K surface temperature for a 3.36d planet orbiting a G8 dwarf? Strange. Perhaps the radius and mass have been reversed? In addition, there are weird inconsistencies between the numbers quoted in the media diagram and in the tables. For example, the diagram pegs WASP-7 at 0.67 Jovian masses, whereas the table lists it at 0.86 Jovian masses. WASP-10 has a period of 5.44 days in the table and 3.093 days in the summary diagram. Putting out a press release without the support a refereed paper is never a very good idea, even when there’s a danger that another team will steal your thunder with an even larger batch of planets.

Despite the difficulty in getting accurate quotes from the exchange, it’s interesting to see how the ten new planets stack up in the transit pricing formula. Using the data from the new WASP diagram (except for the 0.66 day period listed for WASP-9) and retaining the assumption that USD 25M has been spent in aggregate on ground-based transit searches, the 46 reported transits come out with the following valuations:

Planet Value
CoRoT-Exo-1 b $78,818
CoRoT-Exo-2 b $48,558
Gliese 436 b $3,970,811
HAT-P-1 b $883,671
HAT-P-2 b $77,938
HAT-P-3 b $260,473
HAT-P-4 b $172,851
HAT-P-5 b $133,239
HAT-P-6 b $224,110
HAT-P-7 b $54,382
HD 149026 b $722,590
HD 17156 b $869,254
HD 189733 b $2,429,452
HD 209458 b $10,103,530
Lupus TR 3 b $17,488
OGLE TR 10 b $60,260
OGLE TR 111 b $74,524
OGLE TR 113 b $36,599
OGLE TR 132 b $12,326
OGLE TR 182 b $15,261
OGLE TR 211 b $18,653
OGLE TR 56 b $19,761
SWEEPS 04 $1,826
SWEEPS 11 $193
TrES-1 $556,308
TrES-2 $113,043
TrES-3 $93,018
TrES-4 $205,508
WASP-1 $190,539
WASP-2 $188,956
WASP-3 $105,284
WASP-4 $104,581
WASP-5 $65,926
WASP-6 $339,387
WASP-7 $402,125
WASP-8 $209,169
WASP-9 $106,532
WASP-10 $74,281
WASP-11 $233,334
WASP-12 $160,189
WASP-13 $461,104
WASP-14 $14,450
WASP-15 $243,780
XO-1 $436,533
XO-2 $375,996
XO-3 $33,367

The ten new WASP planets (assuming that the correct parameters have been used) contribute about 1/10th of the total catalog value. There will likely be interesting follow-up opportunities on these worlds from ground and from space, but its unlikely that they’ll rewrite the book on our overall understanding of the field.

It’s interesting to plot the detection rate via transits in comparison to the overall detection rate of extrasolar planets. (The data for the next plot was obtained using the histogram generators at the Extrasolar Planets Encyclopaedia, which are very useful and are always up-to-date.)

It’s a reasonable guess that 2008 will be the first year in which the majority of discoveries arrive via the transit channel, especially if CoRoT comes through with a big crop. Radial velocity, however holds an edge in that it’s surveying the brightest stars, and (so far) has been responsible for progress toward the terrestrial-mass regime. I think that we might be seeing planets of only a few Earth masses coming out of the RV surveys during the coming year. Certainly, everything else being equal, a planet orbiting an 8th magnitude star is far more useful for follow-up characterization than a planet orbiting a 13th magnitude star.

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  1. andy
    April 2nd, 2008 at 22:59 | #1

    Further point to add is that WASP-6b was initially reported as a low-mass planet with a large radius (can’t remember what they were exactly but the implied density was down at about 0.16 g/cc). The figures on the illustration are different again, but may be more plausible.

    Shame, a 1.3 Jupiter-mass terrestrial/super-Neptune would have been quite interesting.

  2. andy
    April 2nd, 2008 at 23:12 | #2

    As for the SuperWASP press-release strategy, the last time they announced a bunch of planets (there was a press release announcing WASP-3b,4b,5b in one go), the paper about the first one appeared on arXiv within a few days, but there was a delay of about two months for the papers on WASP-4b and WASP-5b.

    In fact, the figures in the paper about WASP-5b are not the same as the ones displayed on the SuperWASP website (and the Extrasolar Planets Encyclopaedia appears to be using the website figures rather than the paper ones)… what a mess.

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