Ever wondered how science works? Feel you know? If so, read this slowly. There is a puzzle to solve so don’t cheat and read past the question before trying to answer.
WASP 76b is a planet circulating the star known as WASP 76. (WASP stands for “wide angle search for planets” and is an international consortium searching for exoplanets by using robotic telescopes in both hemispheres, hence wide angle. It searches by looking for transits, i.e. a planet passing in front of the star and dimming it. The 76 presumably means the 76th star of interest, and the b means the first planet to be discovered around that particular star.) The star is an F7 class, with a mass of about 1.46 that of the sun, and an effective temperature of about 6,000 oC. So it is bigger, brighter and hotter than the sun.
This planet is weird, by any standards. It is about 0.92 times the size of Jupiter, which means it is a gas giant, and it is 0.033 AU from the centre of the star. (The Earth-Sun distance is defined as 1 AU.) That is close, especially since the star is bigger than the sun. The time taken to go around the star is 1.809886 days. That means a birthday every second day our time, not that there will be anyone having birthdays. The news media has got hold of this because being so close it is expected that the planet is tidally locked. That means, like the Moon going around the Earth, one side is always facing the star and the other side is always facing away. This means that if that is correct and it is tidally locked, the side facing the star will have a temperature of about 2,400 oC, but the side facing away would have a temperature about 1,000 degrees cooler.
When a planet transits in front of the star, the material in the atmosphere absorbs starlight, which gives slightly darker spectral lines, and these give clues as to what is in the atmosphere. In this case, lines corresponding to iron were seen in the gas. At first sight, that is not surprising at 2,400 C. The melting point of iron is 1538 oC, while the boiling point, at our atmospheric pressure, is 2862 oC. It is not hot enough to boil iron, but then again Earth has temperatures that are nowhere near hot enough to boil water, but plenty of water gets into the atmosphere as clouds, and comes down as rain.
This is where the media have sat up and taken notice: it appears it might be raining iron on that planet. That is weird. More evidence was cited for the rain in that the iron signal was unevenly distributed. Recall the light has to go through the atmosphere, so what we see is the signal from the edges. That signal is not evenly distributed, and apparently present on the evening side, but not in the transition edge from night to day. This was interpreted as due to the iron condensing out as it entered the cold side, and there would be liquid iron droplets as rain during the night. Now, here is your test as a potential scientist. Stop reading and think, and answer this question: do you see anything inconsistent in the above description? This is a test for potential theorists. Theories are not developed by brilliant insights, but rather by thinking that something we think is right has an inconsistency.
Anyway, what struck me is the planet allegedly has a morning and an evening. It cannot have that if it is tidally locked, because the same parts always face the star the same way. The planet must be rotating. As an aside, it is hard to see how it could be tidally locked because the gas in the atmosphere will be travelling extremely fast – the winds and storms will be ferocious if there is a thousand degree difference between night and day. But if it is rotating, maybe the difference is not that much. We cannot measure the dayside from transits. Also, if it were tidally locked, we might expect the iron to rain out on the dark side, but then what? How would it get back to the dayside? After a while it would all be on the night side. There has to be some rotation somewhere.Another interesting point is how do you tidally lock gas? And what does rotation of a gas giant mean? In the case of Jupiter we know it rotates because characteristic storms mark the rotation, but Jupiter is far enough from the star that the temperature differences between night and day are trivial. The hot gas around WASP 76b must move. If it is always going the same way, is the planet rotating or merely the gas has a uniform wind?