When we think of the first planets discovered outside the Solar System, we often go back to 1995, when a gas giant was found in a supershort orbit around 51 Pegasi, a star similar to the Sun. So much so that, when the Nobel Committee decided to award the theme of exoplanets, in 2019, it contemplated Michel Mayor and Didier Queloz, responsible for the detection of 51 Pegasi b. However, this was not actually the first exoplanet discovered. The honor goes to two small worlds found in 1992 by Aleksander Wolszczan and Dale Frail, located where least expected: orbiting a pulsar.
It was a shock. Pulsars are stellar corpses that represent what’s left of a high-mass star after it has run out of fuel for nuclear fusion and exploded as a supernova. Any planet the star happened to have had before the detonation would certainly have been devastated. Would they then be second-generation planets, formed after the explosion? Were they captured planets that once roamed interstellar space? When in doubt, it was easier to doubt that the discovery was real. But not only was it confirmed, but in 1994 it was found that there were three planets, not just two, around the pulsar PSR B1257+12.
Time passed, thousands of worlds around “living” stars were found, and to this day we only have a handful of them orbiting pulsars. Which raises the doubt: after all, planets around these dead stars are very rare exceptions or should we find them in droves? A new analysis suggests that these planets are rare indeed.
The work was led by Iuliana Nitu of the University of Manchester in the UK and involved a search for planetary companions around 800 pulsars, using observations from the Jodrell Bank radio telescope spanning nearly two decades. None have been conclusively found.
According to the analysis, two-thirds of pulsars most likely do not have any companion stars with a mass between 2 and 8 times that of Earth. Sensitivity limits dictate that less than 0.5% of pulsars could have planets with at least the mass of the largest of the worlds orbiting PSR B1257+12, with 4 Earth masses. This, however, does not restrict the presence of even smaller planets, such as the smallest of those found in that same pulsar, which has only 2% of Earth’s mass. Such a star would be undetectable in 95% of the analyzed sample of pulsars.
In the end, the group found significant periodicities in 15 of the 800. They could indicate the presence of planets, but in the vast majority of cases they are just an effect of the stellar corpse’s powerful magnetosphere. The team points to a pulsar that is likely to have planetary companions and deserves further investigation: PSR J2007+3120. The work has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
This column is published on Mondays, in Folha Corrida.
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