Doesn't Quite Fit Existing Models of Planet Formation
Max Planck Institute for Astronomy | 2017 Jul 06
Astronomers have discovered a rare, warm, massive Jupiter-like planet orbiting a star that is rotating extremely quickly. The discovery raises puzzling questions about planet formation – neither the planet's comparatively small mass nor its large distance from its host star are expected according to current models. The observations that led to the discovery were made using the SPHERE instrument at ESO's Very Large Telescope. The article describing the results has been accepted for publication in the journal Astronomy & Astrophysics.
[img3="Image of the planet HIP 65426b (bottom left), produced with the SPHERE instrument. SPHERE has physically blocked out light from the central star (blocked-out region marked by circle) in order for the planets much weaker light to become detectable. The light received from the planet allows deductions about its properties – in this case the presence of water vapor and reddish clouds. Credit: Chauvin et al. / SPHERE"]http://www.mpia.de/4337542/zoom-1499239507.jpg[/img3][hr][/hr]Paraphrasing Isaac Asimov, scientific progress is announced not so much by “Eureka!” than by “Hm, this is odd!” The newly discovered planetary system HIP 65426 is a case in point: With a central star in ultrafast rotation, the absence of a gas disk one would have expected for a system 14 million years old and a comparatively light, distant planet, the system doesn't quite fit the existing models for how planetary systems come into being.
Planets are formed in gigantic disks of gas and dust that surround young stars. In the young planetary systems that have been found so far, including all of those observed with the SPHERE instrument, remnants of the disk are usually still visible. There is some degree of correlation in mass: massive stars tend to have more massive disks, forming more massive planets. ...
There is, however, the planet HIP 65426b. Comparing the direct observations with suitable models, HIP 65426b is a warm Jupiter-like planet, with a temperature of about 1300-1600 Kelvin (1000-1300 degrees Celsius), about 1.5 times the radius of Jupiter, and between 6 and 12 times Jupiter's mass. This would make HIP 65426b a gas giant, like Jupiter, with a solid core and thick layers of (mostly hydrogen) gas. Indeed, spectral examinations using SPHERE's spectrograph indicate the presence of water vapor and reddish clouds, similar to Jupiter's. The planet is far out, orbiting its host star at 100 astronomical units (100 times the average Earth-Sun distance, and more than three times Neptune's distance from the Sun).
Again, this represents various levels of oddness: Stars of the type of HIP 65426 (spectral class A2V) are expected to have about twice the mass of the Sun; it has long been assumed that such a star would have much more massive giant planets than the 6-12 Jupiter masses of HIP 65426b. On the other hand, such giant planets would not be expected as far out as HIP 65426b.
Last but not least, the host star HIP 65426 is special, as well: According to spectra taken with ESO's HARPS spectrograph, it rotates about 150 times as fast as the Sun. There is only one other star of similar type that is rotating as fast, and that one is part of a binary star system. In such a system, matter transfer from one star to the other can spin up the receiving star. How a single star could have sped up that much requires an explanation. ...
Discovery of a Warm, Dusty Giant Planet around HIP65426 - G. Chauvin et al