(ORDO NEWS) — Ninety percent of all exoplanets discovered to date (there are more than 5,000 of them) revolve around stars the same size or smaller than our Sun.
Giant stars do not appear to have planets, and this fact has major implications for our understanding of the formation of the solar system.
But is the absence of planets around large stars a true reflection of nature, or is there some kind of bias in the way we look for exoplanets that causes us to miss them? The recent discovery of two gas giants orbiting the giant star µ2 Scorpii (µ2 Scorpii) suggests it could be the latter.
µ2 Scorpii is a star visible to the naked eye – you can go outside and look for it yourself – it enters the tail of the constellation Scorpio, not far from the bright star known as Antares. µ2 Scorpii is a type B star with nine times the mass of the Sun – so large that it will one day explode in a spectacular supernova and then decay into a dense neutron star.
Astronomers recently studied µ2 Scorpii as part of the B-Star Exoplanet Abundance STudy (BEAST) project and found two gas giants – one yet to be confirmed – in orbit around the star. This is the first system of its kind known to us.
Finding these planets was not easy. There are several methods used to detect exoplanets. The transit method allows us to catch planets as they pass in front of their star, causing a brief dip in the star’s brightness from Earth’s perspective.
This method is best for finding planets very close to their star (if a planet takes 12 years to orbit its star, as Jupiter does, it will take 12 years to see the dip again. It is much easier to find stars whose orbits measured in days or weeks).
The radial velocity method, meanwhile, detects planets by watching the star wobble as the planets are gravitationally attracted to it, slightly changing the spectrum of the star’s light to either red or blue. But the radial velocity is also predisposed to finding planets very close to the host star.
Large stars with distant gas giant planets will be easily missed by both the radial velocity method and the transit method.
Fortunately, direct detection of planets is possible in some situations. To do this, the planet must be far enough from its star that it is not drowned out by the overwhelming light of the star. The planet must also be massive enough to be visible, and it must be young enough to be bright (young planets glow hot).
Finally, the entire star system must be close enough to Earth for our instruments to detect them. This is how BEAST was able to detect two planets orbiting µ2 Scorpii, which is part of a star cluster not far from us.
What are the implications of this discovery? This is early evidence that planets of this type are not as rare as previously thought. If BEAST continues to find more gas giants like those around µ2 Scorpii, we will have to rethink what we consider to be the most “common” planetary bodies in the Galaxy.
Moreover, our current models of planetary formation cannot easily explain the formation of a µ2 Scorpio type planet. The accretion model of planetary formation, in which dust slowly accumulates into the planetary core over millions of years, is not possible near massive stars, where protoplanetary disks dissipate faster.
In another model, known as gravitational instability (GI), the protoplanetary disk is massive enough to become unstable under its own weight, collapsing into giant planets.
This could happen much faster than core accretion and could explain planets around massive stars, but µ2 Scorpii’s companion planets, the researchers suggest, “are not expected according to the mass distribution of objects generated by current GI models.” These planets don’t fit the models
Speaking briefly about the significance of this discovery, it becomes clear that the variety of existing exoplanets is greater than what we can currently detect.
Systems like µ2 Scorpii hint at this diversity and will force us to rewrite our models of planetary formation. With each new exoplanet added to our databases, we are learning more and more about the complexity of the solar systems in our galaxy and improving our understanding of the mechanisms at work during planetary birth.
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