An image of a whole planetary system

An international team of astronomers which includes six researchers from MPIA has taken an image showing four planets in the system around the star HR8799. The astronomers made use of the Large Binocular Telescope in Arizona.

20. April 2015
Abb. 1: Direkte Abbildung des Planetensystems um HR8799. Links der Planet HR8799b, auf der rechten Seite, von oben nach unten, die Planeten c, e und d. Der Stern ist mithilfe einer Maske weitgehend ausgeblendet; Reste seines Lichts führen zu den Spuren in der Bildmitte.

To date, almost 2000 exoplanets (planets around stars other than the Sun) have been discovered. Images exist only for a handful of these - the great majority has been detected indirectly. Direct imaging of exoplanets is a challenging task, as the light of a planet invariably gets drowned out by the much brighter light of its host star. Taking an image of a distant exoplanet is the equivalent of taking an image of a firefly that is sitting miles away, right next to a bright floodlight.

Now, a team of astronomers including six scientists from the Max Planck Institute for Astronomy have presented an image of a whole exoplanetary system. The astronomers used the Large Binocular Telescope on Mount Graham in Arizona, and they observed in a carefully chosen range of infrared wavelengths. In this wavelength range, young and hot gas giants (hot versions of a planet like Jupiter in our own Solar System) are comparatively bright, compared to their host star.

The four planets in the star system HR8799, at a distance of 130 light-years from Earth, had been discovered in 2007 and 2010 by direct imaging. Even better, in 2010 another team (also involving MPIA researchers) had managed to obtain a spectrum, a "chemical fingerprint", for one of the planets. The new images, based on raw data taken in 2013, show a snapshot of the four planets' positions. Astronomers have now traced the innermost planet along its orbit for a total of 4 years. For the three outer planets, the available data even spans all of 15 years: After their discovery in 2007, astronomers had been able to identify these planets in images taken in 1998 by the Hubble Space Telescope.

The new image does a particularly good job in imaging regions closer to the star than was previously possible. It confirms the existence of so-called resonances in the planets orbital periods: simple numerical ratios between the times it takes the different planets to complete an orbit. During the time interval it has taken the outermost planet to complete a single orbits, the inner planets will have completed exactly 2, 4, and 8 orbits, respectively. The astronomers could not confirm the existence of an additional gas giant, which would orbit the planet 16 or 24 times faster than the outermost planet; the existence of such an additional planet had been hypothesized on theoretical grounds.

The Large Binocular Telescope consists of two 8.4 meter mirrors on a common mount, making it one of the largest telescopes in the world. German participation is coordinated by the Max Planck Institute for Astronomy.

The results have been published in today's issue of the journal Astronomy & Astrophysics.

Online version of the article describing the results, Maire et al. 2015

Press release of the Large Binocular Telescope Observatory

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