Discovery and naming of Helium

Sir William Ramsay,
the discoverer of terrestrial helium

The first evidence of helium was observed on August 18, 1868, as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India. This line was initially assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D₃ Fraunhofer line because it was near the known D₁ and D₂ lines of sodium. He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, (helios). In 1881, Italian physicist Luigi Palmieri detected helium on Earth for the first time through its D₃ spectral line, when he analyzed a material that had been sublimated during a recent eruption of Mount Vesuvius.

On March 26, 1895, Scottish chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite (a variety of uraninite with at least 10% rare earth elements) with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright yellow line that matched the D₃ line observed in the spectrum of the Sun. These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite in the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight. Helium was also isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, however, attributed the lines to nitrogen. His letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science.

What is Fraunhofer Spectram?

Fraunhofer Spectrum also called as Fraunhofer lines are a set of spectral lines named after the German physicist Joseph von Fraunhofer. The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.

In 1802, a scientist called W.H. Wollaston noticed that the visible spectrum from the Sun had several dark lines in it. Not long afterwards, Joseph von Fraunhofer built the first spectrometer. This focused sunlight from a small telescope onto a narrow slit. The light then passed through a prism, which produced the spectrum. Fraunhofer later invented the diffraction grating, which is used in most spectrometers today. Fraunhofer not only confirmed Wollaston's results, but also found that there were far more dark lines in the spectrum than Wollaston had suspected. Fraunhofer showed that these were a feature of sunlight and not an illusion nor an optical effect, and he labelled them with letters of the alphabet (A,B,C etc.). We now call these dark lines Fraunhofer lines.

What is Plato Telescope?

PLATO is a European Space Agency telescope also called as  Europe's Planet-Hunting Telescope. It is expected to launch in 2024. The name stands for "PLAnetary Transits and Oscillations of stars." The goal of this mission is to figure out under what conditions planets form and whether those conditions are favorable for life. 

To do this, PLATO will seek out and investigate Earth-size exoplanets, especially planets that orbit in the habitable zone around sun-like stars. (The habitable zone is usually defined as the area around a star where there is enough energy for liquid water on a planet's surface, although habitability also depends on other factors such as star variability.) It will determine how big their radii are; verify the mass of the planets from ground-based observatories; use astroseismology or "starquakes" to learn about a star's mass, radius and age; and identify bright targets for atmospheric spectroscopy along with other telescopes. If all goes to plan, the mission should be able to provide detailed information on hundreds of rocky and giant planets, providing more information about how solar systems form generally.

PLATO's primary mission is expected to last four years. However, the telescope is designed to last 6.5 years and its consumables, such as fuel, are expected to last about eight years. This means that the telescope could continue operations if its science mission was extended.

PLATO history

PLATO, which is named after the Ancient Greek philosopher Plato, was first proposed in 2007 after ESA put out a call for its Cosmic Vision 2015–2025 program. Cosmic Vision is the name of the current phase of ESA's long-term space science missions. 

ESA, like NASA, solicits opinions from the science community (ahead of selecting missions) to see what areas of space should be studied next. ESA then puts out calls for missions for a launch opportunity, attracting competitors that must present their science case.

PLATO was first proposed in 2007 as a part of Cosmic Visions, finishing assessment and definition phases in 2009 and 2010, respectively. ESA then put out a call in 2010 for a medium-class mission launch opportunity. 

PLATO, as well as two other missions — Solar Orbiter and Euclid (a mission to investigate dark energy and dark matter) —were selected as the finalists for this competition. Subsequently, Solar Orbiter was chosen for a 2017 launch date and Euclid for a 2020 launch date.

In February 2011, PLATO went up against four other medium-class mission candidates for a 2024 launch date. The others were EChO (the Exoplanet CHaracterization Observatory), LOFT (the Large Observatory For X-ray Timing), MarcoPolo-R (to collect and return a sample from a near-Earth asteroid) and STE-Quest (Space-Time Explorer and QUantum Equivalence principle Space Test). 

PLATO was selected in 2014 for the launch opportunity, which is also called M3 (for the third medium-class mission under Cosmic Visions.) The spacecraft is now in its design phase, which will take several years before it is finalized for construction.

PLATO science

The spacecraft will be launched from Earth on a Soyuz-Fregat rocket bound for a location called a Lagrange point. A Lagrange point is a relatively stable gravitational zone in space. PLATO will specifically be targeted for the L2 Lagrange point, a spot in space on the "dark" side of the Earth (meaning that the sun is always in the opposite direction.) 

L2 has been used before for the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck spacecraft, and is also the region where the James Webb Space Telescope will operate. Since L2 is relatively unstable, the spacecraft will follow a Lissajous orbit, which is a path around the Lagrange point, and periodically use fuel to stay in a consistent orbit.

The payload and science are contributed by a PLATO mission consortium (funded by European national funding agencies) while ESA provides the spacecraft, the CCDs, mission operations and part of the science operations.

PLATO's goal is to watch a large sample of bright stars for months or years, and measure them to high precision. By watching the stars for long periods, PLATO will be able to discern the light curve of the star, or the variations in its light transmitted over time. 

Since PLATO will last four years (at the least), the primary science mission will have it observe two regions of the sky for two years each. It's possible, however, that the telescope could instead do one long-duration observation of three years, and then move around in the sky for the fourth year of its primary science mission. 

"In view of the exceptionally fast development of exoplanet science, the final observing strategy will be investigated throughout the mission development and decided two years before launch," ESA said.

The long-term goal of many planetary observers is to find planets like Earth, and to seek signs of habitability on those other planets. While examining the atmospheres of these tiny planets will require a more advanced observatory, knowing where they are is a first step. 

Other space observatories looking for Earth-like planets include Kepler Space Telescope (in operation since 2009), the forthcoming Transiting Exoplanet Survey Satellite (TESS) and to a lesser extent, the forthcoming James Webb Space Telescope (JWST). Both TESS and JWST should launch in 2018.

PLATO instruments

PLATO has 24 normal cameras on board, arranged in four groups of six. Each of these groups has the same field of view, ESA said, but they are offset by a 9.2-degree-angle from the vertical axis of the spacecraft. Additionally, the spacecraft will have two "fast" cameras that will be used for brighter stars.

If an exoplanet passes in front of a star from the telescope's perspective, it can cause the light from the star to diminish, affecting its light curve. Other things can appear like planets, however, such as sunspots on the star that are darker than the surrounding surface and which also block the light. 

To verify any planets, PLATO will rely on backup observations from ground-based observatories. These observatories can measure the radial velocity of the star, or the velocity of the star along the line of sight from the observer. If slight tugs or movements are seen in the star, this would imply the presence of a planet due to the effect of the planet's gravity on the star.

"The key scientific requirement [is] to detect and characterize a large number of terrestrial planets around bright stars," ESA wrote in a statement. Terrestrial planets, being small, are tough to see around stars because they don't dim the star's light as much. The hope is that by observing closer and brighter stars, the planets will be a little larger and easier to spot.