System Of Three Dead Stars Has Suspected Smallest Exoplanet Ever Found

We know of thousands of pulsars – fast-spinning neutron stars that emit a pulse of radio waves – but so far, PSR J0337+1715 is unique, making it a prime testing ground for Einstein’s Theory of General Relativity. The neutron star itself is fairly normal, but is locked in orbit with two white dwarfs in the dead star version of the 3 Body Problem.

However, Dr Guillaume Voisin of CNRS has suspected for a while that there is a fourth element in this system, a tiny planet that is affecting the intervals between J0337+1715’s pulses. Now, Voisin and co-authors say they have more evidence for its existence, with a mass estimate about half that of the Moon.

Although considered Einstein’s crowning achievement, General Relativity conflicts with Quantum Mechanics, leading physicists on a long quest to reconcile our understanding of the very big and very small. Multiple alternatives to General Relativity have been proposed that would produce indistinguishable results most of the time, but differ under extreme circumstances.

That puts star systems intense enough to serve as test laboratories for General Relativity and its competitors in high demand – and J0337+1715 and its two dwarfs are considered one of the best. 

One of the principles on which Einstein relied was established centuries earlier by Galileo and states that objects in a vacuum fall at the same rate, irrespective of their mass. J0337+1715’s movements can be measured exceptionally precisely because they affect the timing with which we receive its radio pulses. 

The pulsar is locked in a 1.6-day orbit with one white dwarf, and both of them are orbited by another white dwarf every 327 days. Their interactions allow us to measure whether the pulsar and inner white dwarf respond to the more distant dwarf’s gravity identically, as Einstein and Galileo predicted, or if their different masses cause their accelerations to differ. It helps that J0337+1715 is a millisecond pulsar, its signals arriving 366 times a second.

So far, General Relativity has won out, leading physicists to conclude any deviation from equal acceleration must be less than two parts in a million. Other triple-systems containing pulsars are much wider, and therefore cannot test with the same precision.

The detailed observations required for such tests also provide plenty of data that can be analyzed in other ways. In 2020, Voisin and colleagues proposed an apparent pattern in the data might be caused by a very light object on a roughly 3,000-day-orbit. However, they also thought what they were seeing might be a “Red noise process intrinsic to the pulsar emission mechanism”. 

With four more years of data, the team reports the signal is twice as large as the expected uncertainty. They think this rules out one type of red noise. Another type, which might be caused by internal changes within the neutron star, remains possible. However, it would need to be substantially larger than what we have seen in other pulsars.

That makes a low-mass object the most likely explanation, although the authors say only a longer observation period will settle the question with certainty. 

The first planets found outside the Solar System orbit a pulsar, thanks also to the effect their gravity has on the pulsar’s signal. Nevertheless, it’s not easy for a planet to survive the supernova explosion that forms a pulsar without being blown into deep space. Only five pulsar systems are known to have planets, none of them millisecond pulsars.

For a planet to survive such an event while navigating the inherent complexity of a system that once had three suns, all of which have been through the red giant expansion phase, is particularly remarkable. Alternatively, it may have formed recently from debris created in these extreme events.

If the planet exists, the researchers calculate its mass at about 0.004 percent of that of the Earth, which they note means it; “Could be the lightest exoplanet to date according to the extrasolar planet encyclopedia.” Sitting in the mass gap between Pluto and Mercury, it could revive questions about what it means to be a planet. Its orbit would last around eight years and be both highly elongated and at a steep angle to the plane of the system’s larger objects.

The study is published open access in the journal Astronomy and Astrophysics