Space and Physics
PUBLISHED
A long-dead astronomer is nevertheless having a very bad day, after a team of astronomers ruled out his quite cool and reasonable explanation for anomalies in a nearby star system.
One of the great qualities about stars, for any creature out there enjoying the benefits of a long and slow evolution, is that they don't change that much over our timescales. 70 Ophiuchi, just 16 light-years away, was documented by Ptolemy back in the second century CE, and sat there (well, hurtled through the galaxy, as we do) patiently waiting for humans to get their shit together and invent telescopes.
When we did that, and William Herschel observed the star during his work on binary stars in the 18th century, we indeed discovered that the star was in fact two, dancing together in orbit.
At that time, it hadn't been shown that Newton's laws of gravitation applied beyond the Solar System. Herschel's study of these binary pairs and their minute evolution in their position over time showed that stars close together in the sky are not merely floating their nearby, but often gravitationally bound pairs, which adhered to Newton's theory.
But (there's always a but) observations of 70 Ophiuchi had a few anomalies to figure out.
When attempting to figure out their orbits around each other in the 1850s, all hypothetical proposals failed to explain the observations of their true positions. At the time it was expected that Newton's laws of gravitation did apply beyond the Solar System, but it wasn't altogether case shut, and 70 Ophiuchi was a bit of a puzzle in this regard.
"It is just possible that [the discrepancies between proposed orbits and observation] might arise from an erroneous assumption of the universal application of the law of gravitation," William Jacob explained in an 1855 paper.
"[T]his law may be somewhat modified in these remote systems: the general evidence of observation is in favour of the law being everywhere the same as it is within the Solar System, but the data are not exact enough to define this with certainty."
Jacob, by the way, is the astronomer having a disappointing career day in the afterlife (subject to the afterlife being real). The British astronomer proposed that Newton's laws of gravity were correct.
"But there is a simpler mode of accounting for the discrepancy without having recourse to any change of law," he continued. "We may suppose a third body to belong to the system, and to be opaque, and consequently invisible; such a body would, of course, disturb the regularity of the motions of the other two."
Adding a giant planet into the mix, orbiting close to the stars every 26 years, Jacob found that it fit much better with observations, putting discrepancies down to less confident early measurements.
"Disappointment and excitement"
While his paper was perfectly reasonable, and actually quite cool in that it was the first known attempt to use the measurements of the distance of movements of stars to infer the existence of a planet in orbit, the proposed planet has now been ruled out by a new team from the University of Michigan looking at the same star, over 170 years later.
Looking at 499 measurements of 70 Oph A and 334 measurements of 70 Oph B taken between 2023 and 2025, and combining them with previous observations of the system, the team found no evidence for planets orbiting close to the star.
"[T]he present RV [radial velocity] data reveal no evidence for planetary companions around either component of 70 Oph AB, and the observed short-period variability is consistent with rotationally modulated stellar activity," the team explains in their paper.
"Our RV upper limits rule out close-in giant planets in the inner few astronomical units, while the terrestrial-mass regime is beyond current sensitivity."
In fact, the team rules out planets with the mass of Jupiter or larger orbiting the system altogether, using very precise measurements of it to support their conclusions. No other measurements over the last 171 years have been precise enough to rule out a planetary companion of this size.
"It's a mix of disappointment and excitement," Yiting Li, a postdoctoral researcher in the U-M Department of Astronomy, said in a statement. "We're finding out that there isn't a Jupiter-sized planet, but we're also opening the latest chapter on this historic system."
While that's a bit of bad news for Jacob's corpse, the team does not rule out planets of around Saturn's mass, nor the possibility of terrestrial planets, which the team's search was not designed to look for.
"The study sets an upper limit, not a ban, on planets," Li added.
"Given how prevalent planets are in our universe and in the Milky Way, I would wager that we'll find there are other planets there."
The study is published in The Astrophysical Journal.
1