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New planets seen around star

Star like our sun photographed with two exoplanets in orbit

By Andrew ScottPublished 2 months ago 7 min read
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TYC 8998-760-1 with exoplanets 'b' and 'c'. Credits: ESO/Bohn et al.

A revolution is happening, right now.

Galileo completely overturned humanity's previous understanding of the solar system when he saw four moons of Jupiter orbiting the planet.

Now, systems of planets have been seen in orbit around stars other than our Sun. Studying these stellar systems, complete with their planets, will show us if life could evolve elsewhere in the universe. We will also understand more about how our own solar system formed, and what might happen to it in the future.

Until very recently, most detections of 'exoplanets' (planets outside our solar system) have relied on indirect measurement via the Doppler effect:

The Doppler effect in a red dot moving in the direction of the red arrow. Credit: Tkarcher/Tatoute

Objects moving towards the observer reduce the wavelength of light; those moving away increase the wavelength, as seen in the visual below:

An exoplanet orbiting its parent host star, 'pulling' it in different directions. Credit: European Southern Observatory

An observer on earth can see spectral lines from the bright star shifting to shorter and then longer wavelengths as it and the exoplanet orbit a common center of gravity. Even though the exoplanet is not seen directly, its existence can be inferred. Careful study can even show that multiple exoplanets could orbit the same star.

The trouble is, this method doesn't convincingly show that exoplanets exist. Models of how we think stars behave have to be relied on to provide evidence of the exoplanet's existence.

Until now.

The first photograph of an exoplanet in orbit around another star was taken in 2004. The system 2M1207 consists of component 'a', a brown dwarf (too small to be self-sustaining, but still glowing), and its companion 'b', a planet about four times the mass of Jupiter.

Never before then had direct evidence been collected demonstrating the existence of exoplanets.

Since then, that evidence has multiplied, as world-class telescopes have been trained on candidate systems. At the time of writing, there are at least 25 systems for which direct images of exoplanets have been obtained. This list is expected to continue to grow rapidly.

For most of these systems, only one exoplanet has been directly imaged. There might be other planets in the same systems, but they are either too faint to be detected even with very large telescopes, or their light is lost in the glare from the much brighter central star.

Only four years after the first direct image of an exoplanet, the system HR 8799 was photographed, and found to have no less than four planets in orbit around it. The star, of spectral type F, brighter and hotter than our own Sun, is 130 light years away and approximately 50% more massive than the Sun.

The HR 8799 system. Credit: Jason Wang (Caltech)/Christian Marois (NRC Herzberg)

Images taken over a period of time, like the one above, show the four exoplanets in motion. There are three on the right of the star, and one fainter planet to the left. The horizontal bar at the bottom of the image is twenty times the distance from the Earth to the Sun, for scale.

HR 8799 is quite a compact system; the planets orbit comparatively quickly, which is why we can see their motion so readily. It is also really young: 'only' 30 million years old.

It must be quite humbling to witness the motion of planets around another star, an unimaginably long distance away. It's possible that Galileo felt the same when he saw Jupiter's moons through a telescope.

The system TYC 8998-760-1, imaged a few years ago, is one of only a small handful of systems in which more than one exoplanet has been directly photographed. The planets it contains are not as close to the parent star as those on HR 8799, so in the time it has been observed, we cannot directly see the planets moving. At first sight, it doesn't appear to be anything particularly spectacular.

This one is different, though, in one important respect. Observations suggest that the central star's mass is about the same as the Sun.

Never before have exoplanets been directly shown to orbit a star like the Sun.

It needs the pinnacle, the very forefront of modern science and engineering to take the picture at the top of this article.

European Southern Observatory, Chile. Credit: ESO/H.H.Heyer

Each of the reflector telescopes above uses a main mirror no less than eight meters across. It takes a huge telescope like these ones to see exoplanets, not only because they can collect a lot of light and see faint objects, but also so they can resolve planets that are very close to their central star (their angular resolution is inversely proportional to the mirror's diameter).

To improve the resolving power still further, all four of these telescopes above are linked up with optical fibre run underground to large and sensitive instruments. This makes them act as though they have the resolving power of a single huge mirror of the same diameter as the distance between them.

Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE). Credit: ESO/J. Girard

All this light is fed through fibre to one of many instruments. The image of TYC 8998-760-1 was taken by the 'SPHERE' instrument, shown above. Note the engineer on the right hand side for scale. Try fitting that in your back yard!

Detecting the exoplanets requires specialised techniques as well as all this high-end equipment. The star at the center of the system is presently of 'K5' spectral type, approximately the same colour as the one on the bottom right in the picture below, but considerably larger.

A K5 and K7 star, shown in comparison to the Sun - a G2 star. Credit: RJHall

This star would be just too bright to see exoplanets: its glare might also damage sensitive instruments. So a small occulting disk, called a coronograph (so called because the same technique is used to study the Sun's corona or outer atmosphere), blocks out the light from the star. This explains why the star in the image at the top of this article looks so strange.

Another reason the image looks strange is that the light from the star looks 'smeared out'. This isn't a problem with the telescope being out of focus. Even the light from a perfectly in-focus point source is spread out, due to the way in which the optics of the telescope works. This is a fundamental limit, and not due to bad design.

3-D graph of an 'Airy Disk' caused by an ideal Point Spread Function. Credit: Sakurambo

As can be seen in the image above, some (but not very much) of the light from the star is spread out in 'ripples' just like might be seen when a stone is thrown into a pond. This explains the circular pattern around the central star - it isn't real; there's nothing there. Its just the light from the central star, spread out.

What this does mean though is that normally the light from this point spread function must be carefully subtracted from the image to see the exoplanets, which may be very faint and hard to see.

The TYC 8998-760-1, 310 light years away, consists of three known components at this time. The 'a' component is the star. At 17 million years old (which seems impossibly old) this is a mere babe-in-arms compared to the Sun, a hoary 4.5 billion years old. Over time, this large orange K5 star is expected to shrink and become hotter as it reaches the main sequence, at which point it will be just like our Sun, a G2 star.

The first and closest exoplanet, the 'b' component, is approximately 160 times further from the central star as the Earth is from the Sun. It weighs in at a hefty ten to fifteen Jupiter masses, almost hot enough to be considered a brown dwarf in its own right. More recently discovered, the 'c' component is about twice as far out from the central star, and is lighter, at between five and ten Jupiters.

Both the exoplanets in TYC 8998-760-1 are still red-hot, glowing with the residual heat from their formation (gravitational potential energy is converted to heat, or internal energy). This would probably explain why they could be detected. It is thought too that they are in the orbits they formed in - they haven't shifted around.

Why then is TYC 8998-760-1 so significant? Firstly, the exoplanets are well-separated from their parent star. This means that, even though actually they are still incredibly close together, using world-class instruments it is possible to measure the spectrum of dispersed light from each one and work out its chemical composition. The question of how similar they are to the gas giants in our own solar system can then be answered.

It is the second reason which for me is profound, though. These exoplanets are massive - much heavier than the Earth - and very far from their central star. It is unlikely then that they themselves could harbour life. And the system as a whole is still so very young. Life wouldn't have had the chance to evolve since formation.

However, our solar system contains gas giants - as well as the small rocky planets in the inner solar system. On one of those planets, life evolved. Eventually, after billions of years, intelligent life emerged, and knew itself. A civilisation was born.

Other systems like TYC 8998-760-1 probably exist out there. Whose star is like the Sun; whose system contains gas giants. Maybe they also contain small rocky planets, further in towards the star, in the habitable 'Goldilocks Zone'. Old enough to harbour teeming life, and intelligence.

One hundred and fifty years ago, the Prophet-Founder of the Baha'i Faith, Baha'u'llah, wrote:

Know thou that every fixed star hath its own planets, and every planet its own creatures, whose number no man can compute

astronomy
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About the Creator

Andrew Scott

Student scribbler

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  • David Morton Rintoul2 months ago

    We're all fascinating by exoplanets but your story brings them to life. Thanks for the great read.

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