The Road to Characterizing PH1: The Overview
It’s been an exciting week for exoplanets with the discovery of PH1 and the discovery of an Earth-mass planet around the closest star to Earth (Alpa Centauri B). In the coming days and weeks, you’ll hear more about the specifics of how we came out confirming and validating the discovery of PH1 and solidifying that it was a planet orbiting the two central stars in a four star system, but I wanted to give a brief summary of the data and results.
This effort has taken months and months from obtaining the telescope observations, to modeling the light curves, combining Kepler data with radial velocity observations, and applying stellar evolution models. Robert Gagliano and Kian Jek get enormous credit for the discovery and starting this process off and recognizing and spotting the planet transits.
Many collaborators have worked hard to confirm PH1 and study its properties, especially: Jerry Orosz, Josh Carter, Willie Torres, and Debra Fischer who have put tremendous effort (particularly in the past few weeks) to get us from so we found some transits to bona fide planet.
Everyone in this collaboration involved in the paper (including Kian and Robert who were coauthors on the discovery paper) are listed below:
To summarize PH1’s confirmation story, I thought I’d share my press talk slides:
Robert and Kian identified three transits in the light curve of an eclipsing binary . In the binary, there are two stars, one larger and hotter and one smaller and cooler. When the smaller cooler star crosses the face of the larger hotter star, you get some of the larger star’s blocked out and we call that a primary eclipse. When the smaller cooler star crosses behind the larger hotter star, we get a secondary eclipse where the light from the smaller cooler star is blocked out. So we see this big dip+ small dip pattern. Robert and Kian noticed three transits separated by ~137 days in Quarters 1-5 data superimposed on the light curve indicating a third small body in the system, and notified the science team of a possible circumbinary planet.
There’s a small chance that we’re seeing a false positive, where on the sky our Kepler eclipsing binary is aligned with a faint background eclipsing binary giving rise to the transit-like signal. If the transiting body is truly orbiting both stars, we have a way of checking. A body in a circumbinary orbit (orbiting around both stars in a stellar binary), is orbiting a moving target, so the positions and velocities of the two stars are different each transit. So this means that at each transit, there are slightly different gravitational tugs on the planet causing the timing and the duration of the transit to change. We see these changes and this gives us high confidence the planet is really orbiting the Kepler eclipsing binary and not some background source. If you look below at the 7 planet transits across the larger bigger star in the binary, you can convince yourself that the widths of the transits are changing. So bingo, planet!
We got radial velocity observations from the Keck telescopes on Mauna Kea. The radial velocity observations measure the wobble of the larger star in the binary that the planet orbits. With the precision of the observations and time duration we have on the observations, we cannot measure the wobble caused by the gravitational pull of the planet. What we are measuring is the wobble due to the gravitational tug of the smaller star as it orbits the larger star in the binary.
To our surprise we found two velocity contributions in the radial velocity observations. One is from the larger star in the eclipsing binary (solid points) with the model fit for the velocity observations shown in red. The 2nd component is stationary over the roughly 5 months we were taking radial velocity observations. This 2nd component is coming from a source that is providing some light in the spectra slit we place across the Kepler target when observing on Keck. It has the same value as the average or systemic velocity of the binary. So if you take the average value of the black points that’s the velocity of the eclipsing binary (and planet host stars) moving towards or away form us. This second component has the same velocity and random field stars have velocities in the galaxy ranging from ~20-60 km/s so to have a source that has to be very close to the eclipsing binary on the sky that we see it in the Keck observations and have the same average velocity as the eclipsing binary tells us that this source is bound to the eclipsing binary.
We used adaptive optics observations from the Keck II telescope to zoom in and look around our eclipsing binary for other stars that would be contamination the Kepler photometric aperture. We also used deep optical imaging to look for slightly further contamination stars as Bill Keel described in his blog post. As Bill discussed in his post, if there are stars providing extra light to the aperture that is summed up to make the Kepler light curve, it will give us the wrong planet radius. This is because the extra light will decrease the observed transit dips causing us to underestimate the size of the planet. We go these adaptive optics observations while we getting the radial velocity observations.
In the adaptive optics observations there is a source 0.7” away (or about 1000 AU) from the eclipsing binary, and we knew about its existence when we were analyzing the Keck radial velocity observations. There is was the ah-ha moment, where we went oh, this source in the adaptive optics observations must be that second velocity source we see in the radial velocity observations, that we think is gravitationally bound to the eclipsing binary orbiting well outside the planet’s orbit. To our surprise the adaptive optics observations revealed that this source is elongated in one direction (which you can see in the slide below). What we think this means is that we’re just barely resolving the source as a visual binary (2 stars!). So that we have a pair of stars, getting 2 for the price of 1 – (getting us to four stars in the system!) orbiting the eclipsing binary. Our best guess from the Keck observations is that the 2 stars in the distant binary are separated by no more than 40-50 AU.
Combining all of these observations, we went after obtaining the properties of the planet and the stars in the PH1 system. Here below are the properties that come out of the combined photometric-dynamical model that uses the radial velocity observations and Kepler light curve for both PH1 and the stars,. The age we estimate for the entire system from spectral modeling. We estimate the mass of the planet by the fact that it not massive enough to pull on its parents stars sufficiently for us to see slight changes in the timings of when the stars eclipse each other (so when the smaller cooler star crosses the face of the larger hotter star and when the smaller cooler star crosses behind the larger hotter star).
We confirmed with modeling the eclipsing binary properties with just the Kepler light curve, and we’re confident in our estimation for the planet and host star properties. The planet is a gas giant, a bit bigger than Neptune and slightly smaller than Saturn. PH1 orbits its parent stars at a distance between Mercury and Venus in our own Solar System. The planetary system is stable. The planet happily orbits the eclipsing binary every ~138 days not really noticing there’s a second pair of stars out at 1000 AU .
PH1 is the 7th circumbinary planet, and the 6th circumbinary system. Below are the orbits of all the circumbianry Kepler planets and PH1 (not depicting the second distant pair of stars).So why do we care? Well, circumbinary planets are the extremes of planet formation, and we need to understand how they form if we are really understand how we form planets in our own solar system. Planet formation models need to be able to explain both environments, and each of the systems detected gives us another puzzle piece to this picture.