Galileo’s sketches of Venus’ phases. Taken from https://solarsystem.nasa.gov/resources/482/galileos-phases-of-venus-and-other-planets/.

In 1609, Galileo began pointing his telescope at the sky, and one of the first things he looked at was Venus. At the time, of course, it was widely believed that the planets and Sun orbited the Earth, a geocentric cosmology originating before and unfairly attributed to the ancient astronomer Ptolemy. (Ptolemy just published the most famous tables describing the model.)

Galileo’s observations, however, showed that Venus waxes and wanes like the Moon, and it appears smallest in the sky when it is fullest.

Phases of Venus seen from the Earth. Based on https://upload.wikimedia.org/wikipedia/commons/8/8d/The_Phases_of_Venus.jpg.

These observations were hard to reconcile with the assumption that Venus and the Sun circled the Earth, but they made a lot of sense if Venus circles the Sun: Venus was just reflecting sunlight as it orbited the Sun. And so Galileo’s Venus observations provided key evidence for the heliocentric model of the Solar System that eventually supplanted the ancient geocentric model.

We are still using observations of phases to understand planets, but now we can use observations of objects in other solar systems lightyears from Earth. For these extrasolar systems, phase observations can help us determine key properties of the systems and even let us figure out whether we’re looking at a star or planet.

In a recent paper, modern-day Galileos Stephen Kane and Dawn Gelino studied phase curves of planets orbiting a distant star. Unlike for Venus, for the vast majority of extrasolar systems, we can’t easily distinguish the planet’s light from the star’s light. Consequently, when we see variations in brightness from a star, we can’t always be sure whether they are due to a planet, another star, or some other exotic phenomenon.

Instead, as the distant planet circles its star, waxing and waning as seen from Earth, the total amount of light coming from the system goes up and down by a teeny amount, about as much as a firefly flying around a football stadium light.

Brightness variations caused by a planet orbiting a star. Taken from Kane & Gelino (2019).

Big planets orbiting very close to their stars can also induce another kind of brightness variation. If it’s massive enough, the planet’s gravity can distort the shape of the star. As the planet circles the star, these tidal waves on the star rotate in and out of view, making the star brighten and dim.

Tidal distortion of a star (yellow ball) by a planet (small white/black circle), with the resulting brightness variations at bottom.

These variations, called “ellipsoidal variations” after the shape of the distorted star, are bigger for more massive planets, and so we can actually use them to estimate a planet’s mass.

In their paper, Kane and Gelino point out that only planets should exhibit phase variations since stars give off their own light. If we can detect the tiny signals, we can use them to distinguish planets from other objects.

Fortunately, the currently operating TESS Mission will provide lots of scope to apply their technique. In fact, phase variations have already been observed by TESS for the massive, ultra-hot Jupiter WASP-18b.

The first TV picture of Earth from space from the TIROS 1 satellite, 1 April 1960. From https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/wea.550.

On April 1, 1960, NASA took the first weather satellite picture onboard TIROS-1, the Television and Infra-Red Observation Satellite.

In spite of its name, TIROS-1 could only collect images in visible wavelengths, but over its nearly two-month mission, it collected more than 20,000 images of weather systems from an altitude of 700 km, launching the ancient science of meteorology into the space age.

Since those early successes, scientists have found a bewildering variety of weather patterns across the solar system.

For instance, on Venus, a stratospheric jet stream jet stream on steroids herds clouds around the planet every four days, even though it takes almost a year (243 days) for the solid body of Venus to rotate once on its axis (the Earth only takes 24 hours).

A much faster rotator, Jupiter also exhibits jet streams, and, as seems fitting for the largest planet in the solar system, the jet streams are so massive they induce a measurable gravitational signature recently detected by NASA’s Juno mission currently orbiting the king of planets.

A map of Jupiter’s jet streams.

We’ve even managed to extend our meteorological studies beyond the solar system. In 2007, Heather Knutson and colleagues used soon-to-be-decommissioned Spitzer Space Telescope to create a low-res map of the atmosphere of the hot gas giant HD189733b in infrared wavelengths.

Map of HD189733b. From Knutson et al. (2007).

Unlike planets in our own solar system, we can’t directly image HD189733b — it’s too far away. So Knutson and colleagues used a trick to map HD189733b. Since the planet orbits very close to its host star (it circles the star once every 2 days), tidal interaction with the star has probably locked the planet’s dayside to always face the star and the nightside to always face away (reminiscent of the Moon’s rotation).

Knutson and colleagues created their map by simply watching HD189 circle its host star, and as the bright dayside rotated in and out of view, they watched as the amount of light from the planet went up and down. This up and down, called a planetary phase curve, allowed them to decipher which regions on the planet were brighter than others.

And what they found was a little surprising. Even though the planet’s dayside always points toward the star, the hottest place in the planet’s atmosphere wasn’t the place that receives the most sunlight (the “sub-stellar point“) — it was about 3,500 km to the east. Why?

Since HD189 is tidally locked, the dayside is MUCH hotter than the nightside, and this very large temperature contrast drives a super-rotating jet. This jet carries gas through the sub-stellar point at thousands of miles per hour, and by the time the gas has a chance to heat up, it’s already thousands of miles downwind.

Artist’s conception of a hot Jupiter. From https://www.nasa.gov/centers/ames/research/2007/exoplanetweather.html.

But even though jet streams like this can be permanent features in a planet’s atmosphere, they are highly dynamic. For instance, the periodic appearance over the midwestern US of polar vortices result from meanders in the Earth’s jet stream. Since jet streams on hot Jupiters are driven by thousand-mile-an-hour winds, we might expect even more dramatic variability.

Cued by these expectations of jumpy jet streams, our research group studied the planet Kepler-76b, a Jupiter-sized planet zipping around its host star every 1.5 days. The planet is so close to its star that it’s as hot (2200 K or 3500 degrees F) as some low-mass stars.

We analyzed phase curve observations of Kepler-76b collected several years ago by the Kepler Mission, which stared at the planetary system for more than 3 years, which gave us a lot of data to work with.

The phase curve for Kepler-76b from Jackson et al. (2019).

The blue dots in the figure above shows the subtle up-and-down phase curve for Kepler-76b. In our analysis, we found that up-and-down went up a lot during some orbits and much less on others. All told, these variations amounted to changes in the planet’s dayside temperature of hundreds of degrees.

What’s causing this variability? The short answer is we don’t know. But we have some guesses. Previous work has shown that, although Kepler-76b’s dayside is much too hot for clouds (they quickly vaporize), the super-rotation can transport clouds from the nightside.

The cloudiness of a hot Jupiter’s dayside decreases with increasing temperature (left to right). From Parmentier et al. (2016).

We expect that when Kepler-76b’s dayside gets a little cloudy, it will get a little brighter, meaning the phase curve will have more up and down. But since the dayside clouds would now be reflecting more sunlight, that means the dayside will get a little cooler (like on a cloudy day on Earth).

Since the windspeed of the jet stream depends on the temperature contrast between the day and nightsides, a cooler dayside presumably means a weaker jet. The clouds transported to the dayside then evaporate, and the weaker jet cannot bring clouds from the nightside quickly enough to resupply them. Thus, the clouds clear out, the dayside loses its cooling shroud and heats up, which strengthens the jet and starts the cycle all over again.

As in all things science, more studies are needed to decide whether this scenario actually plays out in the atmosphere of Kepler-76b and other hot Jupiter. But for now, studies like ours show that the same kinds of meteorological processes seen on the Earth from space 60 years ago can also be seen on worlds beyond our solar system.

Hot Jupiters are gas giant planets like Jupiter but orbiting so close to their host stars that they can be as hot as small stars. In fact, some suffer such strong irradiation that their atmospheres are being blasted off into space. These objects were among the first exoplanets detected, and their origins and fates remain unclear.

Being so hot, their atmospheres are also unlike any planet’s in our solar system, but we can be certain that, with temperatures hot enough to vaporize rock, the weather on these planets is highly dynamic. The video below shows what happens to one such planet as it gets blast-roasted by its host star – a giant thermal wave screams around the planet.

An animation showing meteorological chaos on HD80606b.

Weather forecasts on hot Jupiters would to have keep track of super-sonic winds, ruby rain, and warm fronts heated by magnetic fields. And a recent study using observations from the Hubble Space Telescope shows that these dynamic atmospheres remain mysterious.

Led by Jacob Arcangeli of the University of Amsterdam, the study used the Wide-Field Camera onboard Hubble to watch WASP-18 as the hot Jupiter circled its star.

As the planet swings revolves around its star, we see first the cool nightside of the planet and then the blistering dayside. By measuring the diurnal cycle of infrared light emitted from the two sides of the planet, called the phase curve, Arcangeli and colleagues were able to measure their respective temperatures and learn something about the planet’s weather.

The phase curve of WASP-18b measured by Hubble (shown in blue), along with model fits.

The figure above shows WASP-18b’s phase curve as blue points and a model fit to the data as a black line. By applying computer simulations for the planet’s weather, Arcangeli and colleagues estimated what they expected the phase curve to look like various assumptions about the planet’s composition and atmospheric dynamics.

A map of atmospheric temperatures (in degrees Kelvin) in WASP-18b’s atmosphere.

Using their models, they were able to draw a rough map of temperatures in WASP-18b’s atmosphere (shown above), like a weather map for the Earth. Like most hot Jupiters observed show far, the hottest place in WASP-18b’s atmosphere lies a little east of the point that receives the most sunlight, the substellar point. That’s because high windspeeds blow the strongly heated gas away toward the east, a little like the jet stream on Earth.

However, the winds inferred from the Hubble observations weren’t as strong as expected from the model, suggesting there is some form of drag or friction in WASP-18b’s atmosphere.

The likely culprit: the planet’s very own magnetic field. Gas in WASP-18b’s atmosphere is actually so hot, it can become somewhat plasma-fied, and plasma, consisting of hot charged particles, can interact with the hot Jupiter’s magnetic field.

This is totally unlike planets in our own solar system, where the atmospheres are comparatively cool and none of the gas has turned into plasma.

The upshot of this result is that we may be able to use observations of the meteorology on distant worlds to learn something about the planets’ magnetic fields, which originate deep inside the planet. So their weather may allow us to plumb the depths of these distant worlds.