This article is published in collaboration with Raman Research Institute (RRI)
Researchers from Raman Research Institute (RRI) and University of Delhi have discovered a massive extrasolar planet (a planet located outside our solar system with a mass that is about 25 times the mass of Jupiter or approximately 8000 times the mass of Earth).
We normally associate planets with objects orbiting ordinary single stars like our sun. However, that is a very limited view. Astronomers have discovered planets orbiting a variety of cosmic entities including exotic ones like compact neutron stars (the remnants of stars that have exploded as a supernova, compact because of their small size but relatively very high density).
BINARY STAR SYSTEM
There are also planets that are in orbit around two stars, which in turn are locked in orbit around each other. Such a two-star system is known as a binary star system. A subclass of binary star system are the X-ray binaries which as the name indicates emit X-rays. One of the stars in X-ray binaries is invariably a neutron star or a black hole which due to its very intense gravitational pull gobbles up star stuff from the unfortunate second component of the binary releasing X-rays in the process.
Now, the natural question that might arise in the mind of the reader is the following: How are astronomers able to discover a planet that is orbiting a binary system located far away?
If the orbital plane of an X-ray binary star system happens to be close to our line of sight towards it, the X-ray emitting star will be eclipsed by the companion star once every binary orbit. The middle of this eclipse, i.e. when the two stars are most aligned along our line of sight, can be measured with high accuracy.
A careful analysis of this mid-eclipse time history helps astronomers to understand the binary system in question. When researchers at RRI studied three distinct X-ray outbursts spanning 40 years from one particular X-ray binary system named MXB 1658-298 they were surprised to find that in addition to orbital decay (due to loss of mass in the companion star) there was also a distinct sinusoidal signature in the eclipse timing residual that is indicative of a third body orbiting the binary system.
Based on calculations involving the masses of the neutron star and the companion star along with their orbital parameters enabled the researchers to estimate the mass of the planet to be about 8000 times that of our earth.
UNIQUE ASPECTS OF THE DISCOVERY
Prof. Biswajit Paul from RRI says “There are several aspects that make this discovery unique:
1. First planet around an X-ray binary stellar system (one of the stars being a neutron star or a black hole emitting X-rays).
2. It is the most massive among the circumbinary planets, about 8000 times that of Earth.
3. This is the most compact of all binary systems around which planets have been found.
4. This is the first planet found using X-ray observations.
5. This discovery is made with a completely new technique, that of periodic delay in X-ray eclipses.”
Researchers presented an X-ray timing analysis of the transient X-ray binary MXB 1658-298, using data obtained from the RXTE and XMM-Newton observatories.
Prof. Biswajit Paul says, “We made 27 new mid-eclipse time measurements from observations made during the two outbursts of the source. These new measurements have been combined with the previously known values to study long-term changes in the orbital period of the binary system. We have found that the mid-eclipse timing record of MXB 1658-298 is quite unusual.”
According to the study “the long-term evolution of mid-eclipse times indicates an overall orbital period decay with a time scale of — 6.5(7) x 10^7 year. Over and above this orbital period decay, the O-C residual curve also shows a periodic residual on shorter timescales.”
“This sinusoidal variation has an amplitude of ~9 lt-sec and a period of ~760 d. This is indicative of the presence of a third body around the compact X-ray binary. The mass and orbital radius of the third body are estimated to lie in the range, 20.5-26.9 Jupiter mass and 750-860 lt-sec, respectively.”
If true, then it will be the most massive circumbinary planet and also the smallest period binary known to host a planet.
For more details visit https://arxiv.org/abs/1703.04433