Astronomers discover a giant planet spinning up its star

Posted: June 7, 2016 by oldbrew in Astrophysics

An artist's image of a hot-Jupiter exoplanet [credit: NASA]

An artist’s image of a hot-Jupiter exoplanet [credit: NASA]

What exactly goes on in terms of interactions between giant planets and their host star? The researchers admit the need ‘to disentangle some of the very poorly understood physics behind tidal dissipation’, as reports. More observations needed.

A giant “hot Jupiter” exoplanet has recently been detected by an international team of astronomers led by Kaloyan Penev of Princeton University. The newly found alien world, designated HATS-18b, is an interesting case of a planet tidally spinning up its parent star.

Moreover, this planetary system could be a great laboratory for researchers when it comes to testing theories of planet–star interactions.

The new findings were presented in a paper published online on June 2 on In order to find exoplanets orbiting HATS-18, the team used the Hungarian-made Automated Telescope Network-South (HATSouth) to obtain over 10,000 images of this sun-like star. This observation campaign was carried out between April 2011 and July 2013.

The astronomers also conducted a series of follow-up spectroscopic observations in 2015, utilizing the 2.3 m telescope at the Siding Spring Observatory in Australia and the 2.2 m MPG/ESO telescope at the European Southern Observatory (ESO) in Chile.

The radial velocity signals collected by the researchers allowed them to distinguish a sinusoidal variation in phase with the transit ephemeris, confirming the presence of a massive planet around HATS-18.

According to the study, the newly detected exoplanet has a radius of about 1.34 Jupiter radii and is two times more massive than our solar system’s biggest planet. The orbital period of HATS-18b equals 0.84 days.

Hot Jupiters

This exo-world is a typical example of a “hot Jupiter”—a gas giant planet with characteristics similar to the solar system’s biggest planet. Hot Jupiters have high surface temperature as they orbit their host stars very closely.

What intrigues the scientists about the newly found planetary system is that the planet appears to be tidally spinning up the star. “The high planet mass, combined with its short orbital period, implies strong tidal coupling between the planetary orbit and the star. In fact, given its inferred age, HATS-18 shows evidence of significant tidal spin up,” the researchers wrote in the paper.

Penev and his colleagues believe that this system could be one of the best laboratories for testing theories of star–planet interactions and planet formation. They noted that modeling this “spin-up” effect for this system alone would bring promising results regarding the tidal dissipation efficiency.

“Such modeling may begin to disentangle some of the very poorly understood physics behind tidal dissipation by measuring its dependence on various system properties. (…) Extremely short-period planets like HATS-18b provide a fantastic laboratory to test a range of interactions between the planet and the star, and hence, expanding this sample is extremely valuable for the study of extrasolar planets,” the paper reads.

Full report: Astronomers discover a giant planet spinning up its star

  1. gallopingcamel says:

    If the star is rotating faster than 0.84 days the planet will be receding as the Moon recedes from the Earth at a rate of inches per year. Otherwise expect the orbit of that “Hot Jupiter” to decay.

  2. oldbrew says:

    HATS-18 b: An Extreme Short–Period Massive Transiting Planet Spinning Up Its Star

    Kaloyan M. Penev, Joel D. Hartman, Gaspar A. Bakos, Simona Ciceri, Rafael Brahm, Daniel Bayliss, Joao Bento, Andr’es Jord’an, Zoltan Csubry, W. Bhatti, Miguel de Val-Borro, Néstor Espinoza, George Zhou, Luigi Mancini, Markus Rabus, Vincent Suc, Thomas Henning, Brian P. Schmidt, Robert W. Noyes, J. L’az’ar, Istvan Papp, P. S’ari

    (Submitted on 2 Jun 2016 (v1), last revised 6 Jun 2016 (this version, v2))
    We report the discovery by the HATSouth network of HATS-18 b: a 1.980 +/- 0.077 Mj, 1.337 +0.102 -0.049 Rj planet in a 0.8378 day orbit, around a solar analog star (mass 1.037 +/- 0.047 Msun, and radius 1.020 +0.057 -0.031 Rsun) with V=14.067 +/- 0.040 mag. The high planet mass, combined with its short orbital period, implies strong tidal coupling between the planetary orbit and the star. In fact, given its inferred age, HATS-18 shows evidence of significant tidal spin up, which together with WASP-19 (a very similar system) allows us to constrain the tidal quality factor for Sun-like stars to be in the range 6.5 <= lg(Q*/k_2) <= 7 even after allowing for extremely pessimistic model uncertainties. In addition, the HATS-18 system is among the best systems (and often the best system) for testing a multitude of star–planet interactions, be they gravitational, magnetic or radiative, as well as planet formation and migration theories.

  3. oldbrew says:

    ‘If HATS-18 did not have a massive planet in such a close orbit, it would have slowed down by this point in its life, rotating around its axis once a month or so. But the planet’s influence makes it spin three times faster’

    So one rotation about every 10 days.

  4. Zeke says:

    Planets modulate their primary main sequence stars. Confirm. (8

  5. oldbrew says:

    ‘Astronomers searching for the Milky Way’s youngest exoplanets have uncovered strong evidence for one unlike any other, a so-called ‘hot-Jupiter’ whose outer layers are being torn away by its parent star.’

    ‘The planet in question, named PTFO8-8695 b, was identified as a candidate exoplanet in 2012 by the Palomar Transit Factory’s Orion survey.

    It is at most twice the mass of Jupiter and is located very close to its host star, the 2 to 3 million-year-old T Tauri star PTFO8-8695, orbiting once every 11 hours.’

    Jupiter orbits the Sun once every 11.86~ years.

  6. oldbrew says:

    Are exoplanet sizes being overestimated?

    ‘Hazes and clouds high up in the atmospheres of exoplanets may make them appear bigger than they really are, according to new research by astronomers at the Space Research Institute (IWF) of the Austrian Academy of Sciences.’

    ‘If the Austrian team are right, this has dramatic implications, for example in the studies of planet populations and how the mass of planets relate to their size.’