Tag Archives: TTVs

Masses for WASP-47’s planets

Since the discovery by K2 of two more transiting exoplanets orbiting WASP-47, plus a fourth planet at a much longer orbital period, this system has shot to the top of the priority lists. A team led by Fei Dai of MIT have thus pointed the 6.5-m Magellan/Clay telescope at WASP-47 to try to measure the radial-velocity signal, and hence the masses, of the two planets found by K2.

WASP-47 radial velocities

The figure shows the complex radial-velocity motion in a multiple-planet system. The blue curve is the radial-velocity motion caused by the 4.2-day-period hot-Jupiter WASP-47b, as originally found by the WASP project. The yellow curve is caused by the 0.79-day planet WASP-47e, the green curve by the 9-day planet WASP-47d, and the purple by the much-longer-period WASP-47c. The red line is then the sum of all the planets and the black dots are the measurements by Dai et al.

By fitting all of the data, Dai et al show that the innermost planet, WASP-47e, has a mass of 12 +/- 4 Earths, while the 9-day planet WASP-47c has a mass of 10 +/- 8 Earths. Both results are in line with previous mass estimates from transit-timing variations, which helps to validate mass measurements by both the RV and the TTV techniques.

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Four planets around WASP-47!

As NASA’s Kepler mission covers fields in the ecliptic previously surveyed by WASP, it is obtaining photometry of unprecedented quality on some WASP planets. The big news this week is the discovery of two more transiting planets in the WASP-47 system.

WASP-47 had seemed to be a relatively routine hot-Jupiter system with the discovery of a Jupiter-sized planet in a 4-day orbit, reported in a batch of transiting planets from WASP-South by Hellier et al 2012.

But WASP-47 is anything but routine. Now Becker et al have announced that the Kepler K2 lightcurves show two more transiting planets: a super-Earth planet in an orbit of only 0.79 days, and a Neptune-sized planet in an orbit of 9.0 days. Being much smaller, these planets cause transits that are too shallow to have been seen in the original WASP data.

WASP-47 transits with Kepler K2

The super-Earth, labelled WASP-47c, has a radius of 1.8 Earths while the Neptune, labelled WASP-47d, has a radius of 3.6 Earths. The triple-planet system is dynamically stable, but the gravitational interaction causes perturbations in the orbits, leading to variations in the times of the transits.

Such “transit-timing variations” or TTVs lead to estimates of the planetary masses. Becker et al find that the hot Jupiter has a mass of 340 Earths (consistent with the mass of 360 Earths originally reported by Hellier et al from radial-velocity measurements), while the Neptune has a mass of 9 Earths. The super-Earth must be less massive than that, but current timing measurements are not sensitive enough to say more.

WASP-47 TTVs Transit timing variations

As if three planets were not enough, there is a probable fourth planet orbiting WASP-47. The Geneva Observatory group routinely monitor known WASP systems, taking radial-velocity measurements over years, to look for longer-period planets. Marion Neveu-VanMalle and colleagues have recently reported the detection of another Jupiter-mass planet orbiting WASP-47, this time in a much wider orbit of 571 days.

The WASP-47 system has now become hugely interesting for understanding exoplanets, and will trigger many additional observations of the system. For example, being bright enough to allow good radial-velocity data, it will provide a much-needed check that the mass estimates from TTVs match those from the more traditional radial-velocity technique.

The system will also be of strong interest to theorists, who will want to understand the formation and origin of a planetary system with this architecture. One immediate consequence is that it shows that a hot Jupiter can arise by inward migration through the proto-planetary disk, without destroying all other planets in its path.