The catch is, they aren't planets- they're stars. White Dwarves to be specific. And the have ten times the mass of the sun in that earth-size package. These two stars represent a class of object we've long known was out there based on mathematics, but now we see them for the first time.
What happens to a star at the end of its life is largely dependent on how large it is in its prime. What happens at the boundaries between categories is where the really interesting stuff happens. Just this year, A discovery made by a 14-year-old student was confirmed, a pint-sized supernova made by a star too big to die quietly but too small to make the giant explosions visible across the known universe that generally accompany supernovae.
But today we're going to focus on the other side of that gap, White Dwarves, the superdense remains of stars too small to go supernova. A typical explanation of a White Dwarf, from Wikipedia:
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy.[1] White dwarfs comprise roughly 6% of all known stars in the solar neighborhood.[2] The unusual faintness of white dwarfs was first recognized in 1910 by Henry Norris Russell, Edward Charles Pickering, and Williamina Fleming;[3], p. 1 the name white dwarf was coined by Willem Luyten in 1922.[4]
White dwarfs are thought to be the final evolutionary state of all stars whose mass is not too high—over 97% of the stars in our galaxy.[5], §1. After the hydrogen–fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf
But what if the star that forms the White Dwarf was larger then our sun? Not too large, mind you, because that would create a supernova, but just a little larger?
Theoretical models suggest that massive stars (around 7 – 10 times the mass of our own Sun) will consume all of their hydrogen, helium and carbon, and end their lives either as white dwarfs with very oxygen-rich cores, or undergo a supernova and collapse into neutron stars. Finding such oxygen-rich white dwarfs would be an important confirmation of the models.
The problem, in practice, is that the collapse of a star small enough to make a White Dwarf doesn't blow away the Hydrogen-Helium shell shielding the star, while the collapse of a star that would be powerful enough to disperse this shell results not in a White Dwarf, but in a Neutron star as a remnant of a supernova. A neutron star is, amazingly in science, perfectly named. Atoms are mostly empty space, its area taken up by the emptiness between a tiny core of neutrons and protons and a huge shell of electrons. Neutron stars blow away those protons and electrons resulting in a superdense 'soup' of neutrons, a single tablespoon of which weighs several tons. Cool, but not easy to see or study.
White dwarves, on the other hand, are brighter. Not massively bright, but bright enough to study:
That little dot in the lower left? Sirius B, in orbit around the more normal-sized Sirius A. But do to the clouds of Hydrogen and Helium mentioned earlier, it's hard to detect details about the star itself.
And to solve that problem, meet the edge cases. Stars in the 7-10 Solar Mass range (seven to ten times the mass of our sun) are right on the cusp, either making very weak supernovae, or, we hoped, making white dwarves but with enough energy to blow away those clouds of other elements.
Unfortunately, almost all white dwarfs have hydrogen and/or helium envelopes that, while low in mass, are sufficiently thick to shield the core from direct view. However should such a core lose its remaining hydrogen envelope, astrophysicists could then detect an extremely oxygen-rich spectrum from the surface of the white dwarf.
And yesterday, it was announced:
Searching within an astronomical data set of the Sloan Digital Sky Survey (SDSS), the University of Warwick and Kiel University astrophysicists did indeed discover two white dwarfs with large atmospheric oxygen abundances.
Lead author on the paper, astrophysicist Dr. Boris Gänsicke from the University of Warwick, said:
“These surface abundances of oxygen imply that these are white dwarfs displaying their bare oxygen-neon cores, and that they may have descended from the most massive progenitors stars in that class.”
Most stellar models producing white dwarfs with such oxygen and neon cores also predict that a sufficiently thick carbon-rich layer should surround the core and avoid upward diffusion of large amounts of oxygen. However, calculations also show that the thickness of this layer decreases the closer the progenitor star is to upper mass limit for stars ending their lives as white dwarfs. Hence one possibility for the formation of SDSS 0922+2928 and SDSS 1102+2054 is that they descended from the most massive stars avoiding core-collapse, in which case they would be expected to be very massive themselves.
You can read up yourself from the Warwick (UK) press release and the journal abstract. Anyone with access to a college library or a couple hundred dollars to buy a personal subscription to the journal Science should read the whole paper, it's quite interesting.