One of my looooooooooooong term projects is monitoring the bright stars in the sky for yet more exoplanets.
These bright stars are some of the very closest Sun-like stars in the sky, and primary targets for next-generation efforts like MINERVA and NEID. Because of their proximity, their planets are the most easily imaged cold planets, the most easily weighed by astrometric measurement (someday), and (even further in the future) the only ones humanity can plausibly target with interstellar probes. We will study these systems for as long as we pursue astronomy.
PSU Professor Eric Ford (but I knew him before he was famous, back in our Berkeley days)
For a few years Eric Ford and I routinely submitted proposals to NExScI to continue monitoring these stars at Keck, and for a while we were successful. It’s a hard sell to a competitive TAC, though, and once Kepler targets started (deservedly) getting lots of NASA Keck RV time, it didn’t make sense to keep proposing.
While it’s not as quick and flashy as Kepler science can be, there are three big ways this sort of patient astronomy pays off: finally getting orbits for long period planets (as in this paper by my student Katherina Feng), monitoring planet-planet interactions among known exoplanets (as in this paper by Ben Nelson), and getting enough data to find new low-mass planets orbiting these stars.
BJ Fulton, University of Hawaii graduate student and author of a nice new paper on some familiar systems.
Well, a lot of that Keck time we invested (combined with a lot of UC and Hawaii Keck time, plus APF time) has paid off yet again. BJ Fulton, a graduate student at Hawaii with Andrew Howard, has just posted his latest paper announcing three Neptune-massed planets orbiting bright, nearby stars. There’s a lot in here!
First, HD 42618: it’s a solar analog (1.05 solar masses) at 24 pc that undergoes a magnetic activity cycle (something we can detect when we monitor stars for decades! It also helps to have Greg Henry’s photometry for this.) There’s CoRoT photometry, so the team was able to extract asteroseismic information to confirm the stellar parameters. The new planet orbits near 0.5 AU, and receives 3x Earth’s insolation from its star. Interestingly, BJ had to remove the stellar activity cycle from the RV time series to get to the planet: the amplitude of the effect is 3 m/s. That’s not abnormally high, but high enough to be annoying. There is also a signal at 2 m/s and P=388 days: this is close enough to 365 days to be both suspicious and difficult to check; if due to another planet, that one is at least 22 Earth masses.
Next up: HD 164922, a G9 star a 22 pc. Here, interferometrists were able to actually determine the radius of the star directly, greatly improving our stellar parameter estimates. Back when I published the Catalog of Nearby Exoplanets, we included in that paper the announcement of HD 164922 b, a P=3-year giant planet. In 2007, as part of my thesis I published a paper in which we noted that we saw a hint of a low amplitude second planet:
— HD 164922 has a known planet with a 3.1 y orbital period. For this star, the FAP
for a second planet is <1%. The best fit for this second planet has P = 75.8 d and
m sin i = 0.06 MJup. The amplitude of this signal is extremely low — only K = 3 m/s —
making this an intriguing but marginal detection.
As luck would have it, this signal was real! BJ finds P=75.8d and K=2.2 m/s — no wonder we had such a hard time picking it out back in 2007! What a difference 9 years of concentrated RV work makes.
The last one is ρ CrB (HD 143761), a V=5 G0 star only 17 pc away. The first planet around this star was detected back in almost 20 years ago (Noyes et al. 1997!) — one of the very first exoplanets. This was back when there was a lot of discussion (too much, in fact) about some or all of the detected exoplanets being face-on stellar binaries. More than once, astronomers pointed to astrometric data suggesting that the signal Noyes et al. saw was from a face-on brown dwarf or stellar companion. Now, this in itself was really interesting, because brown dwarfs were only known for about 2 years when ρ CrB b was discovered! But not as interesting as exoplanets.
Well, not only do we not see any evidence of a stellar companion in interferometry or speckle imaging, but BJ has found a second planet in the system, and he shows that stability constraints make it very unlikely that there is a big old brown dwarf down there — ρ CrB b is almost certainly a planet.
Seeing these planets published is so rewarding—an example of literally decades of work coming to fruition. Seeing these old systems again is sometimes like seeing old friends after a long time and meeting their young families.
You can find the full paper on the arXiv here. You can email BJ with questions or complements about the paper here.