:One solid fact, most experts will agree, is that the planet exists. Its presence was pinned down based on gravitational wobbles induced in the two stars it orbits (the white dwarf and a dense neutron star). That's a reliable method. Still, we at SPACE.com are calling this a "presumed" planet. We'd like to see a picture of it, but technology won't allow that for quite some time.
The suggestion of the planet's age (12.7 billion years) also has a strong foundation.
A key fact is the ancient star cluster in which it resides. The cluster, M4, is known to contain stars that are mostly very, very old. The cluster is thought to have formed about 13 billion years ago and long ago stopped making new stars. The two stars in question are already well along on the path to death and so can be dated. Planets are theorized to form around newborn stars, before the leftover debris of star formation dissipates.
Now, how do they know the planet started out orbiting some other star? Because it could not have formed around the neutron star, which was created in an explosion that would have destroyed any planets. It must have been captured.
"The planet almost certainly could not have originated in that system, hence we believe it was exchanged into its current orbit from a previous orbit around a normal solar like star," the study's lead researcher, Steinn Sigurdsson of Penn State, told us.
An independent expert, planet-formation theorist Alan Boss of the Carnegie Institution of Washington, agreed this is the likely explanation, as do others.
That does not make it firm fact, but the indirect evidence is compelling.
Figuring out where the planet and the white dwarf star came from is trickier. The white dwarf is the remnant of the sun-like star Sigurdsson team thinks was once sole host to the planet. The white dwarf's current tight orbit around the neutron star and data about its mass and emissions provide the clues that suggest -- not prove, but suggest -- that it was once alone with its planet in space, and later was captured by the neutron star.
The suggestion that life that might have once existed in another planet associated with the system is highly speculative. But it is important speculation.
Scientists have long imagined that the raw materials and conditions for planet formation have existed only in, roughly, the second half of the universe's timeline. But if planets could form when the universe was just a billion years old (it's now 13.7 billion years old), then it can't be ruled out that life might have formed back then, too.
It's a staggering notion for scientists to ponder, but as our astute readers point out, it is just a notion for now.
Another reader, Todd, writes: Why do we have to create many layers of speculation to explain an observation? If this situation in some way doesnt "fit" the standard model maybe the standard model is in doubt?
That's a very good question, too. Short answer: The standard model of planet formation is in doubt. So by speculating on this newfound planet's origins, scientists can begin a quest to understand what exactly did happen, and how the standard model should be revised. This will take time and more observations.
Meanwhile, the speculation keeps alive an interesting alternative to the standard model.
For several years now, most planet formation theorists have agreed that the standard model does not explain the presence of Uranus and Neptune. Some new mechanism for planet formation or migration is needed.
Here's what's important:
If the newfound planet is indeed 12.7 billion years old, then it formed in an environment where there were very limited amounts of metals -- the rock and other solid stuff that the standard model requires to build a gaseous planet's core.
Boss, of the Carnegie Institution,
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