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.

NASA’s TESS Mission launched last March and began returning data in the fall. As the worthy successor to the wildly successful Kepler Mission, TESS promises great things.

And already some of that promise is being fulfilled. Astronomers have used TESS data to find exocomets, probe the atmospheres of planets we already knew about, and most recently to find possible evidence for imminent planetary destruction.

The possibly doomed planet is WASP-4b, a gas giant planet that passes in front of its host star every 1.5 days. It orbits so close to its host star that tidal interactions between the star and planet can cause the shape of orbit to vary over time, giving rise to transit-timing variations, illustrated in the video below.

Just this week, Luke Bouma, a graduate student in Princeton astronomy, and colleagues analyzed recent TESS observations of WASP-4b to see if the data show any signs of orbital variations. Not only did they find signs of orbital variation, they found that the orbital period seems to be getting shorter, at a rate of about 12 milliseconds per year, or about one jiffy every year. This about 1000 times faster than slowing of the Earth’s day due to tidal interactions with the Moon.

Variation in WASP-4b’s orbital period as reported in Bouma et al. (2019).

It’s not clear exactly what is causing WASP-4b’s orbit to change, and Bouma explores a couple of options. One idea is that WASP-4b’s orbit is very slightly eccentric and that tidal interactions between the planet and star may be causing apsidal precession, similar to effects experienced by the Moon that complicate the timing of eclipses.

Another, and to my mind more exciting, possibility is that tidal interactions with the host star are drawing the planet inexorably inward. In that case, WASP-4b may eventually be torn apart by its host star’s gravity, a fate that may have befallen many a hot Jupiter.

The great thing about both hypotheses is that they can be tested by additional observations. In fact, amateur astronomers may be able to contribute. Indeed, there is a cottage industry of amateur astronomers observing exoplanet transits, and the hardware, software, and expertise required are pretty minimal for serious amateurs.

Predicted orbital period variations (in minutes) going into the future from Bouma et al. (2019).

Bouma predicts that, over the next several years, WASP-4b’s orbital period might change by several minutes. If the period drops and then increases again (orange curves above), then the variations are likely due to precession, and the planet is probably safe against tidal decay.

However, if the period continues to drop (blue curves above), then the planet is likely doomed to tidal disruption in the next nine million years, short on cosmic timescales.