Artist’s concept of a giant extrasolar planet. Credit: NASA/JPL-Caltech.
The giant planets have masses from 2.4 to 5.5 the mass of Jupiter and have very long orbital periods ranging from nearly 2 to slightly more than 4 Earth years. The team, led by Matias Jones of the Pontifical Catholic University of Chile, made the discovery during observations under the EXPRESS (EXoPlanets aRound Evolved StarS) radial velocity program. They used 2 telescopes located in the Atacama desert in Chile: the 1.5 m telescope at the Cerro Tololo Inter-American Observatory and the 2.2 m telescope at La Silla observatory. Complementary observations were conducted at the 3.9 m Anglo-Australian telescope in Australia.
Using spectrographs mounted on these telescopes, the researchers were monitoring a sample of 166 bright giant stars that are observable from the southern hemisphere. They took several spectra for each of the stars in the sample thanks to these instruments from 2009 to 2015.
Radial velocities of the four giant stars: HIP8541, HIP74890, HIP84056 and HIP95124 show periodic signal variations. The most probable explanation of the periodic radial velocity signals must be the presence of planetary companions. “These velocities show periodic signals, with semi-amplitudes between approximately 50 to 100 ms−1, which are likely caused by the doppler shift induced by orbiting companions. We performed standard tests (chromospheric emission, line bisector analysis and photometric variability) aimed at studying whether these radial velocity signals have an intrinsic stellar origin. We found no correlation between the stellar intrinsic indicator with the observed velocities,” the paper reads.
HIP8541b is the most massive of the newly found quartet of planets. With a mass of about 5.5 Jupiter masses, this exoplanet also has a much longer orbital period than the other 3 worlds, equal to 1,560 days. Its parent star is slightly more massive than the sun and has a radius of nearly 8 solar radii.
HIP74890b and HIP84056b are very similar in terms of mass and orbital period. The mass of HIP74890b is estimated to be 2.4 Jupiter masses, what is about 92% of the mass of HIP84056b. The more massive planet of this comparable duo has an orbital period lasting nearly 819 days – about 3 fewer days than the other planet. Their host stars are also of similar mass and size, about 1.7 the mass of the sun, with a radius of 5.03 (HIP 84056) and 5.77 (HIP 74890) solar radii.
The exoplanet with the shortest orbital period (562 days), is HIP95124b. It has a mass of 2.9 Jupiter masses and orbits a star nearly 2X more massive than the sun, 5.12 solar radii.
The giant planets are preferentially detected around metal-rich stars. “We also present a statistical analysis of the mass-metallicity correlations of the planet-hosting stars in our sample. (…) We show that the fraction of giant planets increases with the stellar mass in the range between 1 to 2.1 solar masses, despite the fact that planets are more easily detected around less massive stars,” the scientists noted.
The team concluded that the high fraction of multiple systems observed in giant stars is a natural consequence of the planet formation mechanism around intermediate-mass stars.
http://arxiv.org/abs/1603.03738
http://phys.org/news/2016-03-giant-planets-stars.htmljCp
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