Back in March, I wrote about how the systemic console could be used to locate a tentative second planet orbiting 51 Peg. The power in the residuals periodogram, and an extensive Monte-Carlo analysis show strong evidence for a Saturn-mass world in a habitable-zone orbit. I was thus quite excited on Saturday when the California-Carnegie group released a heavy-hitting catalog paper that includes (among many other interesting things) 256 re-analyzed Lick Observatory velocities for 51 Peg. These velocity data points are of high quality, with generally small errors.

After using the console to fit out 51 Peg b, the residuals periodogram for all the data still shows a strong secondary peak. The period has shifted, however, to 345 days, and the relative power has declined somewhat with the addition of the new Lick data:

When the residuals are folded at a 345 day period, there’s a visible hint of periodicity:

Note, however, that there are a huge number of points in the right-hand part of the diagram. These are primarily points taken at Lick during the Fall of 1995, just after the Swiss discovery announcement. There’s a scarcity of points near the middle of the diagram resulting from the near-match between 345 days and one year. The star can’t be observed when it’s near the Sun in the sky.

Using the console to get a 2-planet fit, the preferred mass of “c” is still about a Saturn mass, but the eccentricity has increased to e=0.59. This is a worrisome development. The system is still perfectly stable, and an eccentricity of 0.59 is certainly within the range exhibited by exoplanets. The problem is that the fit has adjusted itself so that the pronounced radial velocity swings of the outer planet tend to occur where there is no data.

The Lick data therefore seem to have taken some of the air out of 51 Peg c. It would be good to see whether there’s any hint at all of planet “c” if we consider just the Lick velocities. This is easy to do. Just delete the line containing “51peg_ELODIE04.vels” from the 51peg_B06L.sys file in the datafiles directory that comes with the downloadable console.

Don’t worry if you screw something up. You can toss out a busted console and download a fresh one because they’re free! The only problem with downloading a lot of consoles is that the Internet is not something you just dump something on. It’s not a truck. It’s a series of tubes. And if you don’t understand those tubes can be filled, and if they are filled, when you try to get your new console out, it gets in line and its going to be delayed by anyone that puts into that tube enormous amounts of Internets, enormous numbers of consoles.

When one looks at the Lick data set by itself, the one-planet fit has a required jitter of only 3.6 m/s. This is pretty well in line with the level of astrophysical non-planet noise that would be expected from a star like 51 Peg. There’s really nothing in the Lick-only data to suggest that the model needs a second planet. The strong 350-day residual periodicity is thus present in the Swiss data, but not in the Lick data. In particular, the residuals periodogram to the one-planet fit shows absolutely nothing of interest near 350 days

In my post from last March, I wrote that:

It could very well be true that the 356. day periodicity is due to a systematic effect in the data that has nothing to do with a planet. I would not be surprised at all if this is the real explanation. That is, the observational results might be subject to a small seasonally dependent effect produced by the telescope — as a straw-man example, the temperature of the instrument might have a slight effect on the measured radial velocity. The variation could also have an astrophysical source that has nothing to do with a second planet.

The latest data release from the Lick group certainly seems to favor this conclusion. A telescope-dependent effect is highly unlikely to be present in two independent data sets. The fact that the periodicity is present in only one set of data points toward and Earth-based, rather than a 51 Peg c-based explanation for the periodicity. In the older post, I also wrote that:

As a working scientist, I’ve found that about 95 to 97% of the seemingly publishable “discoveries” that I stumble across end up being spurious for one reason or another. Hope springs eternal, but it always pays to temper one’s enthusiasm!

Continues to be true.

Saturday was an epic day for the radial velocity consuming public. Paul Butler and the California-Carnegie planet search team published a blockbuster paper in the Astrophysical Journal, and it looks like the first weekend’s gross is gonna be huge. The paper announces the detections of five new planets, and publishes re-analyzed and (in many cases) greatly expanded radial velocity data sets for no less than eighty three planet-bearing stars. The velocities are all available in machine-readable tabular form. No dextering, no unfolding, no typing, no postscript extractions. As an added plus, the paper also provides the latest estimates for jitter, mass, metallicity, and vsin(i) for all of the tabulated stars.

Needless to say, there’s a great deal of interesting data in this compendium. The updated 55 Cancri velocities, for example, should aid the characterization of a fully self-consistent model of that system. The slew of fresh velocities will be of great help in constraining the uncertainties in the transit predictions for planet bearing stars.

I dug right in to see how the 51 Peg system (described in a series of posts detailed here) is holding up. There are now 256 new and updated velocities from Lick Observatory to complement the 153 published Swiss velocities. The time-series shows a well-sampled mixture of long-term cadence and intensive monitoring.

Needless to say, 51 Peg b is still present with a vengeance. The power spectrum of the combined 409-point data set has a certain overwhelming 4.231 day character:

The data set phased at 4.2307 days shows a very nice sinusoid. About a thousand orbits have been folded down to make this plot:

So how does 51 Peg “c” fare in the new dataset? I’ll post an analysis tomorrow. If you’re impatient, though, you can use the downloadable console to investigate what the new data has to say.

Image Source.

Several weeks ago, the planet count at the extrasolar planets encyclopedia notched up by one with the announcement by Artie Hatzes and his collaborators that Pollux (Beta Geminorum) is accompanied by a ~2-3 Jupiter mass planet on a 590 day orbit. This world has been under construction for a long time. The first published radial velocity data point for the star dates back to Nov. 15th, 1980, and Hatzes et al. brought 55 new radial velocities to the table to seal the detection. The planet was independently confirmed by Reffert et al., who (in a preprint posted July 7th) deliver an additional 80 high-precision velocities.

All told, there are now seven published datasets, and all are available on both the on-line and downloadable versions of the console. When folded together, at a 593 (1.6 year) period, a full quarter century’s worth of radial velocity data show the planet quite nicely.

After the Sun, Pollux is the 17th brightest star in the sky. It’s prominently visible both because it’s close (34 light years) and also because it’s an intrinsically bright K0III giant star. Pollux is about 1.9 times more massive than the Sun, and is already coming to the end of its life. It has left the main sequence, and is beginning its long trek up the red giant branch of the Hertzsprung Russell diagram.

In last Saturday’s post, I wrote about predictions of the core-accretion hypothesis with respect to planet formation. The ability to quickly build a Jovian-mass planet depends on the surface density of solid material in the protostellar disk. A lot of solids leads to rapid buildup of cores, and hence the ability of planets to achieve rapid gas accretion before the protostellar disk dissipates. (The spiral wave-induced evolution of marginally gravitationally stable disks leads one to expect that disk masses will correlate with the masses of the central stars, see this paper for a lot more discussion.) All other factors being equal, one expects that Jupiter-mass planets will be rarer around stars that have significantly less mass than the Sun, and that conversely, Jupiter-mass planets will be more common around planets of somewhat higher mass than the Sun. (Note that for really massive stars, the luminosity of the star itself will rapidly photo-evaporate the disk, which will cause problems for giant planet formation via core accretion).

Unfortunately, it gets increasingly harder to apply the radial velocity detection method to Main Sequence stars that are considerably more massive than the Sun. The higher temperatures of these stars lead to weaker spectral lines. Weaker spectral lines make it hard to get really accurate radial velocities. Higher mass stars also tend to be fast rotaters, which further smears out the lines, and they are often subject to pulsations which can mimic the radial velocity signature of an orbiting planet. Above about 1.3-1.4 solar masses, it thus becomes hard to survey main sequence stars for planets.

Luckily, however, a trick can be used to assess the planet frequency for high-mass stars. As a star that has ~1.5-3 solar masses ends its main-sequence hydrogen burning life, its core begins to contract and its outer layers swell up and cool down. The atmosphere of the star then regains the wealth of spectral lines that can be used to make accurate radial velocity measurments, and hence detect planetary companions. The core accretion theory predicts that planet hunting around such giant stars should be a highly profitable enterprise.

Image source.

Four out of five astrophysicists surveyed recommend the core-accretion theory to those interested in planet formation theories.

Oklo regulars know that I lean toward core-accretion over gravitational instability as an explanation of the dominant mode of planet formation. I think that core-accretion does a superb job of explaining the planet-metallicity connection, and I don’t think that the initial conditions that underlie hydrodynamical calculations that show disk fragmentation are physically realistic.

The key aspect of core-accretion is that it is a threshold phenomenon. If a planetary core reaches a Neptune-like mass of ~10-20 Earth masses while there is still gas in the protoplanetary disk, then it will rapidly accrete that gas, and (in most cases) increase its mass by a factor of ten or more. On the other hand, if a core reaches a Neptune mass after the gas is gone, then the growth will cut off, and the core will end its days as a modest ice giant.

The amount of time that it takes for a core to reach the phase of rapid gas accretion depends sensitively on the amount of solid material that is available in the disk in the form of planetesimals. A disk with a high surface density of solids is capable of rapidly assembling a core, thereby forming a Jovian-mass gas giant quite quickly. Recent simulations suggest that an average protostellar disk surrounding a star of solar metallicity will lie right at the threshold of being able to manufacture a Jovian planet. This result gives a satisfying mesh with the observations. As stellar metallicity exceeds solar, the fraction of stars with detectable Jovian-mass planets increases very rapidly. Disks that form their Jovian planets early-on are better able to migrate them into the terrestrial region where they can easily be detected. Stars of solar mass and metallicity will tend to have giant planets that remained, like Jupiter, more or less where they formed. Stars with subsolar metallicity will rarely be accompanied by Jupiter-mass planets.

In addition to explaining the planet-metallicity connection, core-accretion provides a number of other testable predictions. Our simulations suggest that a growing planet orbiting a star with 40 percent of the Sun’s mass will require more than 10 million years to “go Jovian.” After 10 million years, however, the gas in most protostellar disks is long gone. The core-accretion theory predicts, therefore, that low-mass red dwarf stars should very often be accompanied by Neptune-mass planets but should almost never have Jupiter-mass companions.

The surface density of solids in a protostellar disk is correlated with metallicity, and some heavy elements are more important than others. Oxygen, for example, in the form of water ice, is of fundamental importance for building cores. At given mass and overall metallicity, therefore, a disk that is naturally rich in oxygen should be better able to form Jupiter-mass planets. Silicon-rich disks too, should have an enhanced capacity for building gas giants.