Beta Pictoris - A young star with a huge dust disk - - The Exoplanet Directory

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Beta Pictoris
August 14, 2016

Beta Pictoris is 63.4 light years away from Earth. It is part of the Beta Pictoris group, an assembly of stars of similar age and comparable velocities and directions of movement, which probably had a common origin [1]. The star is still very young. Its age is estimated to be 8-20 million years [1]. It is 1.75 times more massive than the Sun and 8.7 times brighter [2]. Despite its young age, it has already arrived at the main sequence of stellar evolution (spectral type A6V) [3].

The star is surrounded by an extensive dust disk, which was first observed in 1983 and measures between 1450 and 1835 Astronomical Units in radius. In addition to small particles, this disk is thought to contain many asteroids and comets. It is unusually rich in carbon and especially carbon monoxide. Within the dust disk there are several annular zones with depleted material. It is assumed that in these areas planets have formed or do still form [4].

Distortions of the dust disk indicated the presence of a very massive planet [5]. In fact, the direct optical detection of the planet Beta Pictoris b [6] was achieved in 2008. In 2009 and 2010, the planet was found on the other side of the star, thus demonstrating its affinity to the system. The semimajor axis of the orbit is estimated to be 9.2 AU (+ 0.4 / -1.5 AU), while the orbital time is 21.6 +/- 2.7 years [7]. It is a massive planet of about 7 times the mass of Jupiter (+ 4 / -3) [7]. In line with this, its radius is 1.65 times that of Jupiter [8]. The planet still releases a lot of heat as a result of the compression during its formation: the temperature of its upper atmospheric layer was determined to be 1600 ° C (+ 50 ° / -25 °) [8]. Indirectly, it was possible to measure its rotational speed. The changes in the infrared absorption bands due to carbon monoxide were measured in its clouds. With a rotation time of 8.1 hours, it is one of the fastest rotating planets [9]. The planet is likely to be 5 million years younger than its star. There is even a short video sequence showing the movement of Beta Pictoris b around its sun [10].

By more precise measurements of the position of the planet, the inclination of its orbit could be restricted to 88.8 ° relative to the earth position [11]. As a result, it is not possible to observe the darkening of the star by the planet - an orbital inclination of 90 ° would be necessary for this - but new possibilities of observation are opened up. According to a recently published article, from April 2017 to January 2018 the planet will pass in front of its star so closely that objects in its orbit might cause a dip in brightness. If the planet has larger moons, there is indeed a chance to detect them during this transition [11].

The basis for this is the so-called Hill sphere. This is the area around a planet within which objects are bound by the gravitational force. The Earth's Hill sphere has a radius of about 1.5 million kilometers. Our moon is still within this zone with its mean distance of 380,000 km. Due to its large mass, Beta Pictoris b has a Hill sphere radius of 160 million kilometers, which is larger than the Earth's orbit around the sun. The Hill sphere of Beta Pictoris b will appear in front of the star for the first time in April 2017 and will leave this position again in January 2018. With some luck, moons or other material in orbit around the planet could cause a weak, but still detectable darkening of the starlight. This would be the first and at the same time the best opportunity to observe extrasolar moons.


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