WASP-4 c has a 29-year orbit, while WASP-4 b shows no significant decay

WASP-4 c, a distant giant companion to the hot Jupiter WASP-4 b, has a minimum mass of 9.3 Jupiter masses and an orbital period of about 29 years. Combining measurements of stellar motion, planet transits and the star’s position on the sky also finds no significant orbital decay for WASP-4 b. Simulated Gaia data could sharpen estimates of WASP-4 c’s orbit.

Star system: WASP-4 | Distance from the Sun: 891 light years

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WASP-4 c has a 29-year orbit, while WASP-4 b shows no significant decay

Star system: WASP-4 | Distance from the Sun: 891 light years

WASP-4 c is a distant giant companion with a minimum mass of 9.3 times Jupiter’s and an orbital period of about 10,600 days, or 29 years. The combined measurements also show no significant orbital decay for the hot Jupiter WASP-4 b. Changes in transit times can look like orbital decay, but the travel time of light from a moving star can also shift when the planet’s transits appear to occur.

The analysis combines newly collected radial-velocity measurements from the HARPS instrument with earlier records of WASP-4 b’s transit times. Radial velocity tracks small movements of a star toward or away from us, which can reveal the pull of an orbiting companion. Transit-timing variations are changes in the schedule of a planet crossing in front of its star. The light-travel-time effect describes apparent timing shifts caused by the changing distance light travels as the star moves in its orbit. The same model also incorporates astrometry, which measures a star’s position and motion across the sky.

Simulations of future measurements from the European Space Agency’s Gaia mission indicate that combining all three types of data can improve estimates of WASP-4 c’s properties. In the simulated Gaia Data Release 5 results, the uncertainty falls by about 10 percent for the orbital period, 25 percent for the time of inferior conjunction, and 33 percent for the radial-velocity semi-amplitude. Inferior conjunction is the point in an orbit when the companion passes between its star and Earth. The semi-amplitude measures the size of the star’s back-and-forth motion caused by the companion.

Key findings
WASP-4 c has a minimum mass of 9.3 ± 1.7 Jupiter masses.
Its orbital period is 10,600 days, with an uncertainty of +1,100 and -1,000 days.
No significant orbital decay is found for WASP-4 b.
Simulated Gaia Data Release 5 measurements reduce uncertainties for three WASP-4 c parameters by about 10, 25 and 33 percent.

Scientific study

Combining TTVs, RVs, and astrometry to characterize distant giant companions of transiting hot Jupiters: Application to WASP-4