HATs off to science
The amazing deductions scientists are able to make about exoplanets
If you’ve just read the previous article on the historical significance of the transit of Venus you might be tempted to think that in these days of radar range finding such events would be of little scientific significance, however that’s not the case at all. Transits are still a vital to research though for the most part, the transits scientists are now turning their telescopes towards are not within our own solar system but those taking place across the faces of distant stars.
Dr Daniel Bayliss from the ANU Research School of Astronomy and Astrophysics is one of the lead scientists in the HAT-South Project – an international effort to locate large Jupiter-like worlds orbiting distant stars. Essentially the project comprises a network of telescopes around the globe that between them, are able to monitor a patch of the southern sky round the clock. Data is collected on the brightness of millions of stars and the scientists look for the tell-tale dip in intensity caused by a large planet passing in front of them.
It’s not quite as easy as it sounds though. The odds are stacked against you from the start because to see a transit an observer has to be exactly edge on to the plane of the planet’s orbit. Since the planes are randomly aligned relative to us we miss the majority of such events. Added to that, many stars exhibit periodic brightness variations driven by other mechanisms and just like our sun, most stars also have star spots that can cause minor dips in luminance.
Careful study of a star’s spectrum can usually eliminate inherent variability as a cause of the dips. Likewise star spots can be eliminated if the dips in intensity persist for several cycles.
“Because Earth based astronomers are limited by the atmosphere, the systems we have are really only well suited to looking for large planets orbiting close to their parent stars,” Dr Bayliss says, “But we also have complementary space missions such as the Kepler space telescope that are engaged in a search for smaller Earth-like planets.”
A space based telescope can achieve fantastically accurate photometry (measurements of a star’s brightness) because it isn’t limited by the atmosphere. But it’s not just the size of Earth-like planets that makes them hard to see from the ground.
We expect that such planets would orbit their stars in what’s known as the Goldilocks Zone. Far enough away not to roast but close enough that liquid water can exist. The Earth takes a whole year to orbit the Sun so it would take a distant observer at least four years of continuous photometry to conclusively establish our existence. If the parent star is fairly dim like a red dwarf the Goldilocks zone is closer and the orbits shorter, but they would still take months to complete. Earth based telescopes can only operate on clear nights so for all the HAT-south observatories to be cloud free for years on end is a tall order. However, HAT south transit observations have lead to the discovery of several large Jupiter-like planets in close, fast orbits.
Discovery of such planets is not the end of the science though, it’s just the beginning. “It’s great to find these hot Jupiter systems,” Dr Bayliss says, “But what we really want to know is what the nature of these planets is and how they formed.”
The relative diameter of a transiting planet can be calculated by size of the light dip, bigger planets cause a larger drop. But to translate that proportion into actual kilometres you have to know the diameter of the parent star. That information comes from its luminosity and spectral type. We know for example that a star of the same age and spectral type as the sun that has a slightly greater inherent brightness must be slightly bigger.
Because hot-Jupiter type planets are large they cause the parent star to wobble slightly as they orbit. These wobbles are not necessarily visible as positional changes because the stars are so distant, but they do show up in the spectra as the lines are Doppler shifted by the star’s movement. From the size of the star and the size of the wobble, the mass of the orbiting planet can be calculated. And if you know its diameter and mass you have its density.
“One of the really surprising things we’ve discovered is just how light these planets seem to be,” Dr Bayliss explains, “They’re so light we don’t believe they have the type of rocky core planets like Jupiter do. That makes their existence so much more of a mystery.”
In the absence of rock, it is possible for large gaseous planets to form around accumulations of ice. But ice in such close proximity to a star is a real-life snowball in hell scenario. It simply wouldn’t last for long enough to accumulate its gaseous envelope.
“What we think is happening here is that these large gaseous planets are forming around icy cores beyond what we call the snow line, a distance from the star at which ice is stable. Then some mechanism is driving them inwards.” Dr Bayliss says.
One good candidate for this mechanism is the existence of two or more large planetary bodies in unstable orbits. A close-miss can perturb one planet sending it spiralling inwards towards the parent star. But is there any evidence for the existence of multiple planets in such systems?
The answer is yes. Sometimes they can be detected as additional dips in the light curve but often we’re not that lucky. The chances of both large bodies continuing to orbit in the same plane and hence both transiting from our perspective is very small. Also a large planet in a slow moving distant orbit doesn’t produce nearly such an easily detectable wobble on the central star as it does when it orbits close and fast so they’re generally hard to see that way too.
There is however yet another way large distant planets reveal their presence and that is perturbation of the orbits of inner planets. In fact this is exactly the way in which Neptune was discovered, by the small perturbations it caused in the orbit of Uranus. By looking very carefully at the exact timings of the transits of the closer planet, scientists can sometimes detect small perturbations in the precise moment the light dips. Combining that data over many cycles they can sometimes reveal the tell tale signs of additional invisible planets in the system.
Just when you might think there was nothing else that could be squeezed out of such data, comes the concept of exoplanet weather. Deducing details of the atmosphere of a distant hot Jupiter sounds too fantastic to be true but here’s how it works.
As the planet passes in front of the star and again slips behind it you can see changes in the spectrum. Hot Jupiters have big diffuse atmospheres that are relatively transparent so as they pass in front of their star you can see absorption lines from their constituent gasses. As they go round the back of the star you see the spectral signature of their reflected light disappear. You can use the characteristics of these spectral lines to deduce the temperature and composition of the planet’s upper atmosphere.
Orbital mechanics dictates that such planets are tidally locked, in other words they always keep the same face towards the central star just as our own moon always keeps the same face towards us. This means that one side is intensely heated whilst the other faces the icy cold of space. Now if you look at the spectrum of the planet whilst it’s on one side of the star then look again when it’s on the other it’s possible to deduce how much heat is transferred from the hot side to the cold. And that when combined with the density and composition of the atmosphere tells you something of the winds distributing the heat to the dark side.
Science, especially an observational science like astronomy, is very much like a detective story. Piecing together little clues to build up a picture of what’s really happening. The amazing deductions made by today’s exoplanetary scientists would no doubt impress even Sherlock Holmes.