X-ray and Observational Astronomy

News July 2002: Searching for planets around dead stars

A 30x30 arcsecond infra-red image (J band) of the nearby white dwarf Wolf 1346 taken by the William Herschel Telescope on La Palma with the NAOMI adaptive optics system. The labelled objects might be planets or background stars, later observations will show the movement of those that are planets.	Dotted lines show the expected infra-Red emission spectrum of planets five times Jupiter's mass around nearby white dwarfs. The horizontal lines show the sensitivity of the Gemini and VLT telescopes, demonstrating that planets could be found in the J and H bands.

Stars shine brightly while there is nuclear fuel to burn but fade or explode when it runs out. But while most stars like the sun may live and die over billions of years, eventually fading to become dim white dwarfs, any associated planets can survive almost indefinitely. A new project has started to search for these planets by using the best telescopes available.

Matt Burleigh (University of Leicester) and Fraser Clarke & Simon Hodgkin (University of Cambridge) have calculated the expected brightnesses of large planets around nearby white dwarfs. There are 109 white dwarfs within 20 parsecs of the sun, these are all close enough that planets at Jupiter's distance from the sun could be distinguished from their white dwarf 'sun'. Many possible planets are bright enough, and the white dwarfs dim enough, that using the best ground-based observatories with adaptive optics systems is a good way to search for such planets.

The team have been awarded observing time on the new Gemini North and South 8 metre telescopes (in Hawaii and Chile), and the first data from these observations is now starting to arrive. Because the possible planets are so close the the white dwarf (which is still very much brighter than any planet), it is essential to get images that are as sharp as possible. Atmospheric motion makes stars twinkle when they are close to the horizon because the starlight passes through a large length of air then, the effect is smaller when a star is higher in the sky but is still sufficient to spoil this planet search. Adaptive optics is the name given to the system which removes the effect of the air motions, it works by rapidly sensing the resulting motion of the star's image and making fine adjustments to the shape of the telescope mirror to counteract this effect. Dramatic improvements in image quality can be obtained in this way, making a search for faint planets near white dwarfs possible. The chances of success are also improved by looking in the infra-red region of the spectrum because planets are brightest here while white dwarfs emit more light at bluer wavelengths.

Earlier data from a trial run on the 4.2 metre William Herschel Telescope on La Palma have been processed, and are shown at the top of the page. This image was obtained using the NAOMI adaptive optics system, and shows objects close to the white dwarf as faint as magnitude=20.6 in the J band. Observations over the next few months will show that these are planets if they move together with the white dwarf relative to the more distant stars in the field.

Almost all of the 100 planets beyond our Solar system have so far been discovered by radial velocity measurements which detect the motion of the central star in response to the orbiting planet. These planets can not be directly imaged because this method of finding planets is only sensitive to those very close to their stars, and these stars must be relatively bright for the technique to work. White dwarfs are the end result of the evolution of stars with mass less than 8 times that of the sun, they are are 1000 to 10,000 times fainter than their former selves, and so make good candidates for planet searches.

You may ask 'does a planet really outlive its star?', after all many stars will swell up to about the distance of Jupiter from the sun before they become white dwarfs. Planets engulfed by such red giant atmospheres will spiral in, evaporating as they do so. But stars further out will migrate outwards as the star looses mass via a stellar wind or perhaps a planetary nebula. These planets will survive the death of their star, and for nearby stars, should be visible orbiting the cooling white dwarf.

It is interesting that the first planets found beyond the solar system were discovered around a pulsar. Pulsars are neutron stars, and are formed in supernova explosions that are the end point in the life of massive stars. Did the planets survive such an extreme event? Opinion is devided, but many think that the planets around neutron stars formed from the remnants of the supernova explosion. Neither these planets, nor those around white dwarfs, are likely to be able to support life.

This work was recently reported at a conference on extra-solar planets in Washington DC, and the initial work was published earlier this year in Monthly Notices of the Royal Astronomical Society.

Other news items are available here.

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Last updated: 2002 July 2 by Julian Osborne