10 Feb 2016

T Tauri stars and Extrasolar Planets

The Universe According to Richard Pearson

We are on the eve of a truly historic occasion, because physicists are about to announce the detection of Gravitational Waves (if the recent hype can be believed). I can remember a similar occasion that occurred 30 years ago, and I also sense the rising excitement as we await the announcement on Thursday.
Back in 1981 I had a 15 Cm reflector and my main interest was variable stars. At that time extrasolar planets were in the future, and the Infrared Astronomical Satellite, IRAS, was two years away from launch.

For a number of years there was conversation among amateur and professional astronomers about T Tauri type stars because they displayed a peculiar nature, which was a bit of a mystery.
T Tauri is a variable star in the constellation Taurus. It was discovered in October 1852 by English astronomer John Russell Hind. T Tauri appears amongst the Hyades cluster, not far from Epsilon Tauri. Like all T Tauri stars, it is very young, being only a million years old. Its distance from Earth is about 460 light years, and its apparent magnitude varies unpredictably from about 9.3 to 14.

Importantly, they are generally found near molecular clouds, and T Tauri stars are pre-main-sequence, about 3 times more massive than our Sun, in the process of contracting onto the main sequence as seen in the famous HR Diagram.
Careful analysis of the star’s variable observations suggested that T Tauri itself was associated with a Protoplanetary disk, a young alien solar system in the process of formation. However, there was no evidence, and Infrared astronomy was then in its infancy.

In 1983 IRAS -- The Infrared Astronomical Satellite -- was launched into Earth orbit; it was the first observatory to perform an all-sky survey at infrared wavelengths. It imaged the star Beta Pictoris, and resolved a Protoplanetary disk for the very first time. As infrared techniques improved more discoveries of circumstellar disks followed. So T Tauri itself holds a special place in the history of extra solar planets.
Since 1988 over 2000 exoplanets have been discovered, and with better infrared telescopes, more are being added to the list with the passage of time.

Wanting to learn more about the topic of the discovery of alien planets around distant stars, I have begun reading ‘The Exoplanet Handbook’ by Michael Perryman, published in 2011 by Cambridge University Press. The book is extremely detailed, and covers the main discovery techniques used by astronomers along with analysis of their observations.
In the introduction Michael Perryman notes:

1 Astronomical Unit = the Earth/Sun distance of 93 million miles, or 150 milion Km.

“As of the cut-off date for this review, 2010 November 1, almost 500 exoplanets were known, with more than 50 multiple systems. Diversity continuing the trend established by the ear­liest discoveries, exoplanets does not adhere to the in­dividual or system properties extrapolated from the known architecture of the solar system.
Orbital properties vary widely. Around one third have very elliptical orbits, with e > 0.3, compared with the largest eccentricities in the solar system, of about 0.2 for Mercury and Pluto (and just 0.05 for Jupiter). More than half are around the mass of Jupiter (0.3 - 3Mj), and many of these orbit their host star much closer than Mercury orbits the Sun (0.39 AU): hot highly-irradiated giants piled up towards 0.03 AU that cannot have formed in situ. Others are located far out, at distances of 100 AU or more from their host star. Planets with orbits highly inclined to the star's equatorial plane occur frequently, some even with retrograde orbits.

“Exoplanets are being discovered around a wide va­riety of stellar types. Host stars are not only main se­quence stars like the Sun, but they include very low- mass stars, low metallicity stars, giant stars, and other advanced evolutionary stages such as white dwarfs and pulsars. Their internal structure and composition vary widely too. Gas giants with stripped outer envelopes, water worlds formed beyond the snow line, and carbon- dominated terrestrial planets may all exist. The first Exoplanet atmospheres have been probed through sec­ondary eclipse photometry and spectroscopy.
“Of the multiple Exoplanet systems, massive plan­ets orbiting in mean motion resonance are common, presenting a certain challenge to explain their ubiquity. The first triple-planet Laplace resonance has been dis­covered, as have prominent transit time variations in a two-planet transiting system. Systems with multiple lower-mass planets are being found in increasing num­bers as the radial velocity surveys improve their detec­tion threshold and increase their temporal baseline. The fire-planet system 55 Cnc has been overtaken by the (possible) six-planet system GJ 581 with a 3.1M® planet in the habitable zone, and up to seven planets with five Neptune-mass in the case of HD 10180.”
In my opinion, as we move into 2016 It will not be long before astronomers discover an Earth like planet similar to our own, and then speculation will focus on finding the tell tale signs of intelligent life. The gigantic reflector telescopes now under construction in Chile will help astronomers discover the very first Earth-like planet, and you will then be the first to witness another truly historic occasion.
Richard Pearson

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