At long last, we've discovered planets around normal stars, or had we?
I mean, there was a lot of reasons to be sceptical about these things,
and in fact a large fraction of the astronomical community were really not convinced. There had been too many false alarms in the past.
What we really needed was some independent confirmation.
Exactly, certainly, from my own experience, you need to have two teams
looking at something that's really controversial for people to believe.
In this case, a team in California had come up with a novel idea
to really make sure that they didn't have any, what we call, systematic errors.
Because, if we remember, when you need to measure these tiny, little velocities, any little tiny glitch will give you the wrong answer.
So, what they decided to do is they put a glass cell full of iodine.
Now, iodine has lots of spectral lines, like thousands upon thousands of them,
and they're very narrow, and they put it before the light entered the spectrograph.
As the starlight shone through this iodine cell, it had imprinted on it each of these lines of iodine
which could act as a reference stick for the wavelength of the light.
So, the spectrum you see at the end here would have both the lines from the star itself and the lines from iodine.
Now, the iodine cell is not moving, so if the spectrograph was all perfectly stable, they would always be in the same place.
In practice, if the light lands on different parts of the slit,
or the temperature or the pressure change or any of the other things that can go wrong do go wrong,
that would cause the iodine lines to shift a bit,
but it would shift them by the same amount it would shift the lines of what you were looking at.
That's exactly right, so it was a way of getting rid of all those nasty little problems.
The trick is, these iodine cells are messy, and they really do make a nice looking spectrum
and make it look terrible, so it's very difficult data to analyse.
A lot of hard work had gone into the software. But nonetheless, the Californian group of Paul Butler and Geoff Marcy
had been working on this and, in fact, they actually observed 51 Peg, but they just hadn't analysed the data
because, after all, no one was expecting a planet. We didn't think we could see planets like our own solar system.
But as soon as they heard the result from Switzerland, they, of course, immediately went back
and analysed the data, and they found the same thing.
What's more, when they dug through more of the data, they found more similar sorts of signals.
So, at long last, we had confirmation. We didn't just have one object,
we had a whole bevy of planets suddenly come from essentially nowhere in a very short period of time.
So, here are a few problems we thought of, the first one was "were the data valid?"
The fact that we now have two groups using different methods, so we've got the Swiss group who were using the fibre optic cable method,
and we had the California group using the iodine cell method,
and they both observed the same thing and saw the same velocities at the same time on different telescopes with different methods.
So, that's telling us that yes, these are very different observations, but clearly they're getting it right.
So, Paul, we still have that problem of, potentially, the object just being a brown dwarf nearly face on, so you only get tiny orbital motions.
It could be that the motions are real, in fact, they probably are now that we have two groups finding it,
but maybe it's not a planet, maybe it's a star.
Of course, we've known about binary stars for a long time. But, the more you discover, the less likely that becomes.
Because if you imagine, let's say you have something really massive going around like this,
if it's face on, then most of the wobble is going to be out not in your line of sight,
you're only going to get a very small fraction of it, so it will look like a planet.
But for every one that's going to be face on, there are going to be many more that are at some other angle.
For these ones, you're going to see a huge oscillation, so you'd expect that for every one that looked like a planet,
you'd see your 10 or 20 that look like brown dwarfs, and you wont see anything like that many,
these huge oscillations that could be a brown dwarfs.
So, with just one, you know, maybe it could be a fluke, it happened to be face-on, very close,
but by the time you've got three or four or five, let alone the hundreds we've got now, it doesn't make sense.
Any individual one of them still could be, but as an ensemble, the whole population, no.
Okay, so what about the idea of it being some sort of stellar pulsation,
like a cepheid variable star but something a little different that just mimics the sine wave of a planet.
Well, this was seriously considered, that maybe it was some sort of strange pulsation of the stars surface
rather than the actual whole star moving, the trouble with this is kind of two-fold.
First of all, when you get a pulsation from a star, it very rarely looks like a sine wave.
It tends to be a slow rise and a sharp drop or a steep drop and a slow rise and a slow fall.
Secondly, when stars pulse, they don't pulse at one period. They pulse at multiple periods simultaneously.
They've a resonance, right? They're almost like a musical instrument,
that when you blow your trumpet, it doesn't just give you one tone, it gives you multiple tones.
Like a bunch of harmonics, although they're not very harmonic,
they've all got funny ratios, it would sound horrible if you listened to it. Sort of like bagpipes.
Bagpipes and bells are two instruments that have harmonics that aren't nice at regular intervals.
But people only see the one sine wave, so that made it very hard to come up with any model of a star that would do that.
Okay, so it looks pretty solid.
What were they discovering? Well, most of the ones they were discovering were something like this, the hot Jupiter,
where a planet's in a near circular orbit very close in. This is a schematic, it's not really to scale.
To scale it looks more like this. There's a dot coming across. (That's less impressive.)
Much less impressive, which is why most people don't give you scale drawings of these things.
So, were all the objects going in these close circular orbits?
The bulk of them were, but there was also another class of things,
what we call the 70 Virginis systems after the first one discovered, 70 Virginis.
These ones, the planets are much further out, this one again is not to scale,
they're actually maybe as far out as the Earth, an astronomical unit out, but they're very elliptical orbits.
They came very close then whipped around very close and then way out slowly then in and close.
So, you can really see them when they come close, the way you find these things is the fact that they are elliptical
and they have big radial velocities here and very tiny ones out there.
That's right, at least to begin with, that's how they were discovered.