Three-faced star: Exoplanets in the infrared

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  • Published: Feb 1, 2017
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Three-faced star: Exoplanets in the infrared

The youthful HR 8799

Jason Wang and Christian Marois have animated astronomical observations of the star HR 8799 and its four exoplanets to give humanity a first glimpse at an alien solar system

There is a star in the constellation of Pegasus that is 129 light years from Earth. It is a young whippersnapper just 50 million years old (compare that to the sun's cosmic elder statesmanship of about 4.5 billion years. This star, HR 8799, has planets, hot planets far bigger than Jupiter and Saturn that swing around it in vast orbits, almost chaotic orbits. NASA's Spitzer Space Telescope first sighted the system as the Keplerean chaos kicked and jostled comet-like bodies, leading to a huge halo of dust, a circumstellar disc, around the system visible in the infrared.

Needless to say astronomers and planetary scientists have been watching and recording activity in this distant system ever since 2009 when scientists analysed an archived NICMOS image and spotted a likely exoplanet. By 2011, a NICMOS (The Hubble Space Telescope's "Near Infrared Camera and Multi-Object Spectrometer") image from 1998 had unearthed three exoplanets around the star. A fourth massive planet was identified in 2010.

Planetary "gif"

Scientists at the National Research Council of Canada’s Herzberg Institute of Astrophysics have been among those collecting and analysing data from this distant solar system over the last few years. Now, Jason Wang at the Nexus for Exoplanet System Science (NExSS) has aggregated images into an astonishing "video" showing the movement of the exoplanets in the HR 8799 planetary system. Wang's video, or perhaps it might be termed more precisely a stop-motion animation, has hit the headlines recently giving humanity the first vision of planets moving around another star in their own "solar" system.

The orbits shown in Wang's video are incomplete, of course, not least because scientists have only been watching the system since 2009 and even the closest planet to the star has an orbital period of about 40 earth years. By contrast, the most distant planet takes 400 earth years to undertake a complete circuit of its star. In other words, it will be many years before Wang's animation is complete. However, given the inherent chaos in this distant solar system and its youthful vigour it is likely that those orbits will shift as time goes on and future generations of astronomers will watch this cosmic tale unfurl in a fascinating way.

Of course, the system is not entirely chaotic, Wang and colleagues think that the four planets may be moving in resonance with one another. There is a one-two-four-eight ratio in which the orbital period of each individual planet is related to the others.

Three-faced enigma

While observations of any exoplanets can help us better understand our own to some degree and provide useful clues as to how stellar systems form and evolve, HR 8799 (its Bright Star Catalogue name) is not like our own Sun. The star has characteristics of three other types of star. Primarily, it is described as a Gamma Doradus variable wherein its luminosity undulates because of non-radial pulsations of the star's surface. In addition, HR 8799 might also be classed as a Lambda Boötis star, because its surface layers are much depleted in iron peak elements, perhaps due to the accretion of metal-poor circumstellar gas. These two phenomena make HR 8799 uniquely the only star we know that is a Gamma Doradus variable and a Lambda Boötis type. But, that circumstellar disc which glows with excess infrared also makes the star a little bit like Vega too, so it is not so much a two-faced star as a triptych enigma.

Related Links

Many Worlds, 2017, online: "A Four Planet System in Orbit, Directly Imaged and Remarkable"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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