UHerts, UCSC: Earth-sized planets detected orbiting the nearest sun-like star

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MargaritaMc
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UHerts, UCSC: Earth-sized planets detected orbiting the nearest sun-like star

Postby MargaritaMc » Wed Aug 16, 2017 7:27 pm

Two potentially habitable planets detected orbiting the nearest Sun-like star
https://www.herts.ac.uk/about-us/news/2017/august/two-potentially-habitable-planets-detected-orbiting-the-nearest-sun-like-star
A new study by an international team of astronomers, led by the University of Hertfordshire, reveals that tau Ceti, the nearest Sun-like star (about 12 light years away from the Sun), has four Earth-sized planets orbiting it – two of which could be habitable.
These four planets have masses as low as 1.7 Earth mass, making them among the smallest planets ever detected around the nearest Sun-like stars. Two of them are super-Earths located in the habitable zone of the star and thus could support liquid surface water.
These planets are detected by observing the wobbles in the movement of tau Ceti. The newly discovered planets correspond to detecting variations in the movement of the star as low as 30 cm/s, whilst 10 cm/s is the upper limit required for detecting an Earth analog - a planet or moon with environmental conditions similar to those found on the planet Earth.

Milestone in the search for Earth analogs
Dr Fabo Feng, research fellow at the University of Hertfordshire and lead researcher on the study, said: ‘We’re getting tantalisingly close to observing the correct limits required for detecting Earth-like planets. Our detection of such weak wobbles is a milestone in the search for Earth analogs and the understanding of the Earth’s habitability through comparison with these.’



Four Earth-sized planets detected orbiting the nearest sun-like star
https://news.ucsc.edu/2017/08/tau-ceti-planets.html
A new study by an international team of astronomers reveals that four Earth-sized planets orbit the nearest sun-like star, tau Ceti, which is about 12 light years away and visible to the naked eye. These planets have masses as low as 1.7 Earth mass, making them among the smallest planets ever detected around nearby sun-like stars. Two of them are super-Earths located in the habitable zone of the star, meaning they could support liquid surface water.

The planets were detected by observing the wobbles in the movement of tau Ceti. This required techniques sensitive enough to detect variations in the movement of the star as small as 30 centimeters per second.

"We are now finally crossing a threshold where, through very sophisticated modeling of large combined data sets from multiple independent observers, we can disentangle the noise due to stellar surface activity from the very tiny signals generated by the gravitational tugs from Earth-sized orbiting planets," said coauthor Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz.
[...] The data were obtained by using the HARPS spectrograph (European Southern Observatory, Chile) and Keck-HIRES (W. M. Keck Observatory, Mauna Kea, Hawaii).

A paper on the new findings was accepted for publication in the Astronomical Journal and is available online. In addition to Vogt, Feng, and Tuomi, coauthors include Hugh Jones of the University of Hertfordshire, UK; John Barnes of the Open University, UK; Guillem Anglada-Escude of Queen Mary University of London; and Paul Butler of the Carnegie Institute of Washington.


Color difference makes a difference: four planet candidates around tau Ceti
Fabo Feng, Mikko Tuomi, Hugh R.A. Jones, John Barnes, Guillem Anglada-Escude, Steven S. Vogt, R. Paul Butler

https://arxiv.org/abs/1708.02051
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Re: UHerts, UCSC: Earth-sized planets detected orbiting the nearest sun-like star

Postby Ann » Wed Aug 16, 2017 11:22 pm

You know me, Margarita, I'm more interested in the star than in the planets (although I will be more interested in them when we know more about them).

So, Tau Ceti. I was surprised to hear it described as a Sun-like star, because that is not how I have been thinking about it. I have been thinking about it as one of all those stars in the Sun's vicinity that are less massive and less luminous than the Sun. And I was right about that, because according to Wikipedia, Tau Ceti has only 78% of the mass of the Sun and about half its luminosity (slightly less than half the Sun's luminosity in visual light).

But even so I was surprised that Tau Ceti is as luminous as it is, given its relatively low mass. So I decided to compare it with Alpha Centauri B, the K1V-type component of Alpha Centauri. Alpha Centauri B has almost 91% of the Sun's mass, but like Tau Ceti, it is about half as luminous as the Sun. So Tau Ceti really is relatively luminous for its mass, or otherwise Alpha Cen B is faint for its mass.

But the biggest surprise, when I compared Tau Ceti with Alpha Centauri A and B, was the color indexes of these stars.

Alpha Centauri A: Mass, 1.100 M. Luminosity, 1.519 L. B-V = +0.71. U-B = +0.24.

Alpha Centauri B: Mass, 0.907 M. Luminosity, 0.5002 L. B-V = +0.88. U-B = +0.68.

Tau Ceti: Mass, 0.783 ± 0.012 M. Luminosity, 0.52 ± 0.03 L. B-V = +0.72. U-B = +0.21.

Now that is weird! Tau Ceti is more light-weight than Alpha Centauri B, but it has almost the same color indexes as much more massive Alpha Centauri A, and it is much bluer and more ultraviolet than Alpha Centauri B! How weird!

So when it comes to its spectral characteristics, Tau Ceti is pretty Sun-like. It resembles the Sun in that it is a single star, too. But anyway, the more I look at the properties of stars, the more I think that we shouldn't take our Sun for granted. Its properties may be rarer than we think.

Ann
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MargaritaMc
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Re: UHerts, UCSC: Earth-sized planets detected orbiting the nearest sun-like star

Postby MargaritaMc » Thu Aug 17, 2017 4:01 pm

This info might help, Ann.

• It is known that metallicity affects the colour of stars, with low metallicity stars being bluer.
• The alpha Centauri system has high metallicity.
• Dr Kaler notes that tau Ceti has low metallicity. (As does the Wikipedia article on tau Ceti)

Here are some links.
https://en.wikipedia.org/wiki/Metallicity
Metallicity also affects a star's color temperature. Metal poor stars are bluer and metal rich stars are redder.


http://stars.astro.illinois.edu/sow/taucet.html
Tau Ceti also stands out as a modestly high-velocity (37 kilometers per second) local visitor from the "thick disk" of the Galaxy that surrounds the thin disk that makes the Milky Way. Older, the thick disk has a lower metal content, Tau Ceti's about half that of the Sun.


https://arxiv.org/abs/1202.1265
Oscillations in the Habitable Zone around Alpha Centauri B
Duncan Forgan
(Submitted on 6 Feb 2012)
The Alpha Centauri AB system is an attractive one for radial velocity observations to detect potential exoplanets. The high metallicity of both Alpha Centauri A and B suggest that they could have possessed circumstellar discs capable of forming planets.


An unrelated observation is that it is intriguing that rocky planets have been detected around a low metallicity star.
Last edited by MargaritaMc on Thu Aug 17, 2017 4:30 pm, edited 1 time in total.
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
— Dr Debra M. Elmegreen, Fellow of the AAAS

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MargaritaMc
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Re: UHerts, UCSC: Earth-sized planets detected orbiting the nearest sun-like star

Postby MargaritaMc » Thu Aug 17, 2017 4:25 pm

Sky & Telescope article
Four Exoplanets Might Orbit Sun-like Star
By: Monica Young | August 15, 2017
Astronomers have used an innovative technique to discover four super-Earth-size exoplanet candidates orbiting Tau Ceti, a Sun-like star 12 light-years away.
http://www.skyandtelescope.com/astronomy-news/four-exoplanets-may-orbit-nearby-sun-like-star/
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
— Dr Debra M. Elmegreen, Fellow of the AAAS

The What not the Who

Re: UHerts, UCSC: Earth-sized planets detected orbiting the nearest sun-like star

Postby The What not the Who » Sat Aug 19, 2017 3:01 pm

Good stuff.


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