Posted by Many years ago, as I was writing up my thesis, my advisor burst into my office with the hot news.
Someone had announced a possible discovery of an extrasolar planet!
It seems strange now, but back then we really did not know if there were any other planets around other stars.
A lot of astronomers thought that it was quite likely that there were planets around other stars, maybe even most stars, but we had no data.
We were getting there, there were astrometric and radial velocity searches underway, with people like Walker, Griffin and Latham developing techniques which would clearly, eventually, maybe, in a decade or two, tell us about the presence of Jovians.
New high precision radial velocity surveys were being set up, in particular by Mayor in Switzerland, and Marcy in California.
Nature had published the possible detection of a low mass companion around HD114762, which is either a low mass brown dwarf or a high mass Jovian, depending on how you look at it.
Several other claims of exoplanet detection had been published, and then retracted, generally because of unsuspected systematic errors, or understandable but slight overinterpretation of marginal data.
PSR B1829-10 was exciting, it was different, the data was good. 10 Earth masses, few month orbital period!
For some hours I contemplated suspending work on my thesis, which was on companions to pulsars and how they might come to be there, and doing something, anything, on this discovery.
Then, I played with some equations, and sent a cryptic e-mail to my advisor, noting that the signal looked worryingly like a buried second harmonic of the Earth’s orbit.
There were some e-mails between people above my paygrade, several people had noticed this, and then the retraction came.
The Solar System barycenter had not been correctly subtracted from the radio wave data, and the detection was invalid.
The retraction and handling of the related publicity, by the way, was exemplary – and precisely how discovery retraction ought to be well handled.
But, the stage had been sent, in particular the understanding that radio pulsar measurements were precise enough to detect planets – not just giant planets, but Earth mass planets.
A few months later, Alex Wolszczan announced the discovery of PSR B1257+12 with the first confirmed extrasolar planets orbiting it.
This was a tour de force of astronomical discovery.
Two planets, with masses of about 4 Earth masses, and third planet found later in the data, with a mass of 1/50th that of the Earth. That is still the lowest mass exoplanet found, and the sensitivity of pulsar searches remains about three orders of magnitude better than the current best optical radial velocity searches. Pulsar data has sensitivity equivalent to velocity variations of mm/sec!
This was real, and in quick order the predicted planet-planet orbital perturbations were seen in the data.
There was a proliferation of theory papers, as always, coming up with increasingly outlandish theories for how a pulsar, and a recycled millisecond pulsars at that, could possibly have planets.
There is now a good model for the formation, by Hansen and collaborators, with the planets formed within a narrow annulus of metal rich debris evolving viscously, probably from fallback from the supernova. This has implications, not just for formation channels for millisecond pulsars, but also for terrestrial planet formation in general (see also here).
A workshop was hastily put together to discuss the pulsar planets.
It was held at the end of april 1992, at Caltech. I was invited, and told to put something together for the meeting.
So I did: Planets in Globular Clusters.
Globulars are full of pulsars, good for timing, and there’s lots of stars. Maybe we could find planets there – globular stars are also low metallicity, but a quick calculation showed planets might still form in the more metal rich clusters, although maybe with lower efficiency than for metal rich stars, and the pulsar planets might form through a different mechanism from regular planets.
I showed 1257+12 planets might form and persist around cluster pulsars, though they might become dynamically perturbed by neighbouring stars; and, that a different channel existed where pulsars could steal planets from normal solar like stars, and that these would have a qualitatively different orbital distribution from planets formed in situ.
The meeting was very exciting, the Rodney King riots broke out the day before the meeting started. We drove down from Santa Cruz on the 101, into LA listening to the World Famous KROQ and the sudden traffic advisories about not exiting the freeways near downtown… The deli around the corner from the condo we stayed in was shot up that night, and the ambient atmosphere was a bit tenser than normally in SoCal.
At the meeting, Don Backer grabbed me, and showed me the data on PSR B1620-26 (Steve Thorsett had previously tagged the system in his PhD thesis as having anomalous timing behaviour) – it was immediately obvious that there could be a long period jupiter mass planet in the system, and I started arguing the case for such.
For several years, starting at the 1994 Aspen Center for Physics meeting on Pulsars, a minor debate raged over PSR B1620-26 – the initial solution was consistent with anything from a sub-jupiter mass planet to a black hole!
But as more data came in, the constraints tightened until it was clear the object orbiting in the system had to be sub-stellar.
Then a fortuitous encounter over a good south coast pinot noir at a Kavli Institute meeting lead to the realization that serendipitous Hubble data existed for the field, and in quick order this provided independent confirmation that the second pulsar planet was there.
The oldest planet, and still my favourite
It also confirmed, what we were beginning to suspect, that planets really are ubiquitous.
Not a problem.
I am also sure that more pulsar planets will be discovered.
There are a lot of interconnects in astronomy.
In 1999 a group of us, lead by Ron Gilliland and Tim Brown, got some (ok, a lot of) Hubble time to look for transits of planets in a globular cluster – 47 Tuc specifically.
(Don’t know what fool suggested that:
“Observation is most probable in the rich high-density precore collapse clusters such as 47 Tuc.”).
We found no planets in 47 Tuc…
Which was a puzzle. Which I mentioned to my then graduate student.
We did get a lot of practise doing high precision relative photometry, showed that space-based relative photometry really had the necessary precision, and that false positives due to grazing binaries, variables and blends could be eliminated with high confidence.
A decade later, the student even figured out why – Solar Tides. The planets close enough to the star for the Hubble search to have detected them (two transits less than 4 days apart), would have been destroyed, not by the tide of the star on the planet – we checked that! – but by the subtler tide of the planet on the star, the slowly rotating old star.
Nice piece of work.
The original exoplanet discovery has been somewhat overshadowed by the later discovery of exoplanets around solar type stars – that was pretty exciting also, but that is another story.
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