NASA launches satellite ‘TESS’ in hunt for exoplanets

With the launch of NASA’s TESS satellite due this very day, this is a popular-level account of TESS and exoplanet hunting that I wrote for The Conversation (and which has been re-published by the BBC Focus Magazine). Actually this is my version, prior to their editing.

Previous generations have looked up at the stars in the night sky and wondered whether they are also orbited by planets; our generation is the first to find out the answer. We now know that nearly all stars have planets around them, and as our technology improves we keep finding more. NASA’s newest satellite, TESS (the Transiting Exoplanet Survey Satellite), scheduled for launch on Monday, will extend the hunt for small, rocky planets around nearby, bright stars.

NASA’s TESS planet hunter (artist’s impression)

We want to know how big such planets are, what orbits they are in, and how they formed and evolved. Do they have atmospheres, are they clear or cloudy, and what are they made of? It is conceivable that, over coming decades, we might find a planet that has a size and mass similar to Earth’s. If that planet also lies at the right distance from its star for water to be liquid, and has an atmosphere containing molecules such as free oxygen and has an atmosphere containing molecules such as free oxygen then we could even find biological activity. TESS is a major step towards that long-term goal.

Planets are so faint and tiny compared to their host stars that it is remarkable we can detect them at all, let alone study their atmospheres. TESS exploits the fact that, if the planet’s orbit is edge on to us, then it will “transit” across the face of the star, blocking a small fraction of the star’s light. TESS will monitor 200,000 bright stars in the solar neighbourhood, looking for tiny dips in their brightness that reveal a transiting planet.

During transit, the planet’s disc is surrounded by the thin smear of its atmosphere, back-lit by the bright star. That starlight will be absorbed at some wavelengths by molecules in the planet’s atmosphere, while at other wavelengths it shines straight through. By observing the starlight passing through the planet’s atmosphere and splitting it up into its spectrum, we can deduce what that atmosphere is made of.

The spectrum of starlight passing through a planet’s atmosphere can tell us what the atmosphere is made of. Credit: Christine Daniloff/MIT, Julien de Wit

Such observations are right at the limit of current capabilities, requiring the James Webb Space Telescope (JWST), the $8 billion successor to Hubble scheduled for launch in 2020. With a 6.5-m mirror, collecting much more light than Hubble, and with specially designed instruments, JWST has been built to study exoplanet atmospheres.

With JWST time being so valuable we first need to know which stars have the best transiting exoplanets to study, and that’s why we need TESS. Its predecessor spacecraft, Kepler, surveyed 150,000 stars in a patch of sky near the constellation Cygnus, and found over a thousand planets ranging from gaseous giants like Jupiter to rocky planets as small as Mercury. But Kepler covered only a small patch of sky, containing few bright stars, so most of the stars it looked at are faint, and that makes it hard to study their planets.

In contrast, ground-based surveys for transiting exoplanets have searched wider swathes of the sky looking at brighter stars. The most successful has been the UK-led Wide Angle Search for Planets, of which I am a member. Using an array of camera lenses, the WASP project has spent the last decade monitoring a million stars every clear night looking for transit dips. Nearly two hundred planets have been found, and indeed many of the transiting exoplanets so far chosen as JWST targets come from WASP.

But ground-based transit surveys have one big limitation: they look through Earth’s atmosphere, and that severely limits the data quality. They can detect brightness dips of 1 per cent, which is sufficient to find giant gaseous planets like our own Jupiter and Saturn, but smaller, rocky planets block out far less light. Our Earth would produce a dip of only 0.01 per cent if seen projected against our Sun.

The JWST is currently being readied for launch. Credit: NASA/Chris Gunn

TESS will combine the best of both, observing bright stars over the whole sky, but with the advantage of space-based photometric quality. It should find the small, rocky planets that Kepler proved are abundant, but transiting much brighter stars. That means they can be studied and characterised by JWST.

TESS will observe each region of sky for typically 30 days, meaning that it will be most sensitive to planets with short orbital periods that produce frequent transits. Planets in short-period orbits are close to their star. Therefore most of the planets that TESS finds will be highly irradiated and too hot for liquid water. But if found orbiting red-dwarf stars, which are intrinsically dim, even close-in planets can be in the habitable zone.

The dwarf star TRAPPIST-1 is a thousand times less luminous than our Sun, and is known to host seven short-period planets. TESS will look for such planets around other dwarf stars, and while TESS does so from space, the SPECULOOS survey will be attempting the same from the ground. All such finds will be prime targets for JWST.

Looking further ahead we then want to find rocky planets in the habitable zone of stars like our Sun. They will have longer-period orbits and so will be harder to find. That is the task of the ESA satellite PLATO, currently under development for launch in 2026. The race will then be on to find biomarker molecules in the atmosphere of an Earth-twin exoplanet.


“Sharing Reality” and how to persuade people

How does one best persuade people to favour a secular and science-based view of life? That’s the topic of Jeff Haley and Dale McGowan’s new book: Sharing Reality: How to Bring Secularism and Science to an Evolving Religious World (of which the authors kindly gave me a review copy). [ link; link]

They start by discussing how not to do it. They quote Neil deGrasse Tyson’s remarks to Richard Dawkins:

“And it’s the facts plus the sensitivity [that], when convolved together, create impact. I worry that your methods, and how articulately barbed you can be, end up simply being ineffective.” It’s significant that Tyson didn’t complain that Dawkins’s approach was unpleasant or disrespectful. He said it was ineffective. His argument is that Dawkins’s own presumed goal of convincing others that his ideas are worthy and important is short-circuited by a failure to consider the state of the mind on the receiving end of those ideas.

It’s a common complaint, that Dawkins is too acerbic and dismissive of religious opinion, appearing to talk down to people. For example, Emily Willoughby writes: Continue reading

The cosmological multiverse and falsifiability in science

The cosmological “multiverse” model talks about regions far beyond the observable portion of our universe (set by the finite light-travel distance given the finite time since the Big Bang). Critics thus complain that it is “unfalsifiable”, and so not science. Indeed, philosopher Massimo Pigliucci states that instead: “… the notion of a multiverse should be classed as scientifically-informed metaphysics”.

Sean Carroll has recently posted an article defending the multiverse as scientific (arXiv paper; blog post). We’re discussing here the cosmological multiverse — the term “multiverse” is also used for concepts arising from string theory and from the many-worlds interpretation of quantum mechanics, but the arguments for and against those are rather different. Continue reading

How not to defend humanistic reasoning

Sometimes the attitudes of philosophers towards science baffle me. A good example is the article Defending Humanistic Reasoning by Paul Giladi, Alexis Papazoglou and Giuseppina D’Oro, recently in Philosophy Now.

Why did Caesar cross the Rubicon? Because of his leg movements? Or because he wanted to assert his authority in Rome over his rivals? When we seek to interpret the actions of Caesar and Socrates, and ask what reasons they had for acting so, we do not usually want their actions to be explained as we might explain the rise of the tides or the motion of the planets; that is, as physical events dictated by natural laws. […]

The two varieties of explanation appear to compete, because both give rival explanations of the same action. But there is a way in which scientific explanations such as bodily movements and humanistic explanations such as motives and goals need not compete.

This treats “science” as though it stops where humans start. Science can deal with the world as it was before humans evolved, but at some point humans came along and — for unstated reasons — humans are outside the scope of science. This might be how some philosophers see things but the notion is totally alien to science. Humans are natural products of a natural world, and are just as much a part of what science can study as anything else.

Yes of course we want explanations of Caesar’s acts in terms of “motivations and goals” rather than physiology alone — is there even one person anywhere who would deny that? But nothing about human motivations and goals is outside the proper domain of science. Continue reading

Human rights rest only on human advocacy

Are human rights anything more than legal conventions? asks John Tasioulas, Professor of Politics, Philosophy and Law at King’s College London.

Isn’t the answer “obviously not”? Human rights are collective agreements, statements about what sort of society we want to live in, and of how we want people to be treated. As such, their justification and standing derives from the advocacy of people. Anything more than that is mere rhetoric, “nonsense on stilts”, as Jeremy Bentham explained long ago.

But, as with morals, people get unhappy about the idea that their feelings on the matter are all there is. People would really like human rights to be laws of nature, objective obligations that we ought to follow regardless. Wouldn’t that put them on a sounder footing? Continue reading

Another philosopher of science doesn’t understand science

Maybe I’m having a philosopher-bashing week. After disagreeing with Susan Haack’s account of science I then came across an article in the TLS by David Papineau, philosopher of science at King’s College London. He does a good job of persuading me that many philosophers of science don’t know much about science. After all, their “day job” is not studying science itself, but rather studying and responding to the writings of other philosophers of science. Continue reading

Science is a product of science!

The latest issue of Free Enquiry magazine contains several articles about philosophy and science, including an article by Susan Haack, a philosopher of science who “defends scientific inquiry from the moderate viewpoint”, rejecting cynical views that dismiss science as a mere social construction, but also rejecting “scientism”.

While Susan Haack talks quite a bit of sense about science, she promotes a view that is common among philosophers of science but which I see as fundamentally wrong. That is the idea that science and the scientific method depend on philosophical principles that cannot be justified by science, but instead need to be justified by philosophy. Continue reading