Saturday, December 26, 2015

Possible Planet Orbiting the White Dwarf PG 0010+280

PG 0010+280 is a young white dwarf with an estimated surface temperature of 27,220 K and a cooling age of approximately 16 million years. This object is the dense leftover core of what was once a star with roughly 1.8 times the mass of the Sun and with a main sequence lifetime of about 2.3 billion years. PG 0010+280 is estimated to have 57 percent the mass of the Sun and a diameter of roughly 20,000 km (~1.6 times the diameter of Earth).

Observations of PG 0010+280 reveal an infrared excess at the 3 to 8 μm waveband. This infrared excess can be attributed to the presence of either an opaque dusty disk orbiting the white dwarf within its tidal radius or a blackbody with a temperature of roughly 1,300 K. If the infrared excess is due to the presence of an opaque dusty disk, the inner edge of such a disk can be as close as ~380,000 km from the white dwarf. Any closer than that, the intense radiation from the white dwarf can sublimate the dust grains.

If the infrared excess is due to a ~1,300 K blackbody, such an object will have to be about 1.3 times the diameter of Jupiter. This is consistent with either an irradiated substellar object (i.e. brown dwarf) or a re-heated giant planet. Assuming the object is an irradiated substellar object and assuming that it is in thermal equilibrium with the incoming radiation from the white dwarf, such an object will need to be ~220,000 km from the white dwarf in order for its temperature to be ~1,300 K. At that distance, the irradiated substellar object will have an orbital period of 21 hours.

Before becoming a white dwarf, a star will swell into a red giant and eject its outer layers. Any giant planet around the star can be significantly re-heated from the accretion of material ejected from the star. For a giant planet around PG 0010+280, it only needs to accrete a tiny percentage of the ejected stellar material for it to be significantly re-heated to ~1,300 K. Such a process can involve the accretion of a substantial amount of carbon-rich material that can cause the planet’s atmosphere to be noticeably enriched in molecules such as methane (CH4) and carbon monoxide (CO).

Siyi Xu t al. (2015), “A Young White Dwarf with an Infrared Excess”, arXiv:1505.02614 [astro-ph.SR]