Gaia

DECCAN HERALD 3

Tuesday, February 21, 2017

Spectrum science

FORMED AT AN EARLY STAGE

Anewestimateofasolarnebula’slifetimesuggeststhatJupiter&Saturnmust haveformedwithinthefirstfourmillion yearsofthesolarsystem’sformation.

Making a map of Milky Way

STellar Surveyor rishabh Shukla elaborates on the features of Gaia, a space telescope that will help us understand the origin and evolution of our galaxy

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hen the European Space Agency (ESA) released a catalogue containing precise positions and brightnesses of 1,142 million stars of the Milky Way galaxy on September 14, 2016, it was a treasure trove of information for astronomers. Gaia, a spacebased telescope launched on December 19, 2013 by the ESA, is on its mission to study the evolution of our galaxy. As of today, it is observing approximately one billion celestial bodies including stars, planets, comets, asteroids and quasars in the Milky Way, building the most accurate three-dimensional map of our galaxy we have till now.

A billion celestial bodies make just one per cent of our galaxy’s population — a scale unimaginable to most of us. Nevertheless, the tremendous data generated

by Gaia will be used for determining the positions and motions of stars (astrometry), measuring the colours of the stars (photometry) and measuring the radial velocity (spectroscopy) and studying the constituents of stars.

The name Gaia is derived from ‘Global Astrometric Interferometer for Astrophysics’ (GAIA), an electromagnetic waves-based measurement technique that is used in the telescope. However, the acronym GAIA has been dropped and now the name Gaia remains. The project was proposed by Professor Lennart Lindegren of Lund University, Sweden and Dr Michael Perryman of the ESA in October 1993, and was authorised by ESA in 2006.

But why do we need to study stars? The history of attempting to map the night sky is as old as time. The ancient Greeks, Babylonians, Mesopotamians and Indians were sophisticated astronomers and used elaborate clay tablets, detailed star charts and star maps for mapping the positions of stars and planets. These stellar positions were used to calculate direction, time and seasons.

Unprecedented precision Though the curiosity to study the night sky has remained, the process is now aided with sophisticated telescopes and space probes which give accurate and up-to-date information on celestial objects to improve our understanding of the universe. However, the night sky is not as calm and tranquil as it looks — it is peppered with countless stars bursting into life and imploding to their death, all invisible to our eyes.

With technological advancements and space probes like Gaia, the origin, structure and evolution of these stars and galaxies can be uncovered.

The ESA’s first space mission dedicated to mapping the sky was called Hipparcos and was launched in the year 1989. ‘Hipparcos’was an acronym for High Precision Parallax Collecting Satellite, and it also reflected the name of the ancient Greek astronomer, geographer, and mathematician, Hipparchus, who is considered as the founder of trigonometry and the discoverer of the precession of the equinoxes.

It catalogued nearly 1,20,000 stars in its lifetime. Gaia is the successor of Hipparcos and is extending this effort by measuring 10,000 times more stars and other celestial objects and their motions.

With a mission lifetime of five years, Gaia would have observed each star in the galaxy over 70 times. “Besides directly mapping the night sky, Gaia will measure distances to one billion stars in the Milky

A VAST EXPANSE Besides directly mapping the night sky, Gaia (inset) will measure distances to one billion stars in the Milky Way. REPRESENTATIVE IMAGE

Way with unprecedented precision. To work out distances of objects outside our galaxy, astronomers rely on what is known as the ‘distance ladder’. Measurements of distances to far off quasars or galaxies depend on knowing distances to nearby objects within our galaxy accurately,” comments Dr Vimal Simha, a postdoctoral researcher at the Centre for Extragalactic Theory, South Africa. It will also be able to measure all of these with unprecedented precession, he adds.

So where in the sky is Gaia placed? In order to measure the position of stars with high precision, Gaia has to be positioned in a gravitationally stable orbit. There are five such naturally occurring points where the net gravitational force of the Sun and the Earth cancel each other. They are called Lagrangian points, named after the 18th century Italian mathematician and astronomer, Joseph-Louis Lagrange.

Gaia is positioned at Lagrange-2 (L2) point which lies 1.5 million km from the Earth in the opposite direction to the Sun and co-rotates with the Earth around the Sun. Other advantages of the L2 orbit is that it offers uninterrupted, eclipse-free observations with the ability to observe the entire celestial sphere during the course of one year. Gaia contains two telescopes pointing towards two different patches of the sky, 106.5 degrees apart.

At the heart of the telescopes are Charge Coupled Device (CCD) detectors that are sensitive to a single photon of light. This high sensitivity records stars that are 400 thousand times fainter than the stars visible to naked eye. To measure the distance of stars in relative to each other, Gaia measures the parallax angle of the stars. Parallax is the effect whereby the position or direction of an object appears to differ when viewed from different positions.

Stars with high parallax angle are nearer and those with low parallax angle are far. Gaia aims to map every object in the sky with a resolution of 20 micro arcseconds, enough to see a waving hand on the moon from the Earth. Although Gaia’s mirrors are not large enough, they are polished to up to 10 nanometres of accuracy.

Since there is no atmosphere in space to

With a mission lifetime of five years, Gaia would have observed each star in the galaxy over 70 times.

blur the image, Gaia captures clear images with high resolution.

Stellar catalogues Stellar catalogues are to astronomers, what a map is to sailors. “A stellar catalogue, as the name suggests, is a systematic list of physical parameters of stars. Gaia will produce a catalogue containing positions, brightnesses and colours for a billion stars in the Milky Way. Moreover, because the positions of stars will be measured many times, astronomers will be able to work out the speed and direction of motion of stars,” explains Dr Vimal. To carry out this complicated endeavour, ESA has constituted the Data Processing and Analysis Consortium (DPAC), a team of 450 scientists and software experts who will analyse the data generated by Gaia.

Most of the stars are so far away from the Earth that it is impossible for us to send a spacecraft and study them in our lifetime. Hence, astronomers rely on clues like radiations emitted by stars and their colour to study their mass, temperature, age, composition and other properties.

“Gaia’s principal objective is to make the most detailed map ever of our galaxy. This will help us understand the origin and evolution of the Milky Way galaxy. For example, it will help us understand some phases of evolution of stars that are, at present, not so well understood because of the paucity of data,” adds Dr Vimal. Its mapping can also help astronomers use it for other things such as to look for extrasolar planets and massive explosions known as supernovae in other galaxies, he adds.

Researchers across the world are already taking advantage of the data that is released and are working on project proposals that can harness this information.

“The first release of Gaia data, available on the Gaia archive, is already being used for projects along the lines of understanding our galaxy’s evolution and its constituents,” says Dr Vimal.

This endeavour will surely open new avenues in the field of astronomy, and shed some light on some of the fundamental questionsabouttheexistenceofMilkyWay. (The author is with Gubbi Labs, a Bengaluru-based research collective)

Gene editing: an ethical minefield once-unThinkable Scientists are divided on the proposition to modify human embryos to create genetic traits that can be passed down to future generations, writes amy harmon

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n influential science advisory group formed by the National Academy of Sciences and the National Academy of Medicine, USA on February 14, 2017 lent its support to a once-unthinkable proposition: the modification of human embryos to create genetic traits that can be passed down to future generations. This type of human gene editing has long been seen as an ethical minefield.

Researchers fear that the techniques used to prevent genetic diseases might also be used to enhance intelligence, for example, or to create people physically suited to particular tasks, like serving as soldiers. The advisory group endorsed only alterations designed to prevent babies from acquiring genes known to cause “serious diseases and disability,” and only when there is no “reasonable alternative.”

The report provides an explicit rationale for genetic research that the US federal government has avoided supporting until now, although the work is being pursued in countries like Sweden and China. Socalled germline engineering might allow people to have biological children without fear that they have passed on the genes for diseases like Huntington’s and without discarding embryos carrying the mutations, as is often done now.

‘Thisisn’tpossible,sowedon’thavetothink about it much,’”said Richard Hynes, a cancer researcher at the Massachusetts Institute of Technology, USA, who helped lead the committee with Alta. “Now we can see a path whereby we might be able to do it, sowehavetothinkabouthowtomakesure it’s used only for the right things and not for the wrong things,”he said.

A more pragmatic concern driving the committee was the likelihood that the new technology would be adopted, in countries like China, where some pioneering research on editing human embryos — without the intent to gestate them — has already occurred. “If we have an absolute prohibition in the United States with this technology advancing, it’s not like it won’t happen,” Alta said.

Push to engineer traits?

But opponents of human germline editing say that is not a reason to take a crucial step towards what they fear will be an inevitable push to engineer traits like strength, beauty and intelligence, perhaps eventually creating a dystopian social divide between those who can afford such enhancements and those who cannot.

“This opens the door to advertisements from fertility clinics of giving your child the best start in life with a gene-editing packet,” said Marcy Darnovsky, executive director of the Centre for Genetics and Society, a public interest group in Berkeley, California, USA. “And whether these are real advantages or perceived advantages, they would accrue disproportionately to people who are already advantaged.”

In addition to social concerns, there are also questions of safety and autonomy.

While CRISPR is generally precise, it can have “off-target” effects, cutting DNA at places where it is not meant to. The new report called for prohibiting any alterations resembling “enhancement”, including “off-label” applications. Under the guidelines, a genetic technique aimed at strengthening the muscles of patients with Duchenne muscular dystrophy, for instance, could not be used to make healthy people stronger. But it is not clear who would draw those lines.

No one should expect to design a baby anytime soon. It will most likely be several years before gene-editing techniques tested in animals can be shown to work in humans. And for the moment, the Food and Drug Administration is prohibited from using federal money to support research that results in genetically modified offspring.

In the meantime, said Sharon Terry, the president of the Genetic Alliance, a patient advocacy group, patients who may participate in clinical trials must begin a conversation. “My hope is that there would be serious considerations about what we are balancing here,” she said.

Assessing risks better The new report heralds a day scientists have long warned is coming. After decades of science-fiction movies, cocktail party chatter and college seminars in which people have idly debated the ethics of humanity intervening in its own evolution, advancing technology dictates that the public now make some hard choices. “It is essential for public discussions to precede any decisions about whether or how to pursue clinical trials of such applications,” said R Alta Charo, a bioethicist at the University of Wisconsin-Madison, USA and a leader of the panel that wrote the report. “And we need to have them now.”

Just over a year ago, an international group of scientists declared that it would be “irresponsible to proceed”with making heritable changes to the human genome until the risks could be better assessed and until there was “broad societal consensus about the appropriateness” of any proposed change. No one is pretending that such a consensus now exists. But in the year that the committee was deliberating, Alta said, the techniques required to perform this sort of gene editing have passed crucial milestones.

The advent of a powerful gene-editing toolcalledCRISPR-Cas9allowsresearchers to snip, insert and delete genetic material withincreasedprecision. Ithasled to plans for experimental treatments of adult patientswithcancer,blindnessandotherconditions as early as this year. But these types of genetic alterations are not inherited.

“Previously, it was easy for people to say, The New York Times

ALTERING DNA A gene-editing tool called CRISPR-Cas9 allows researchers to snip, insert & delete genetic material with increased precision. REPRESENTATIVE IMAGE

The hunt for rogue planets just got tougher M

ost planets live their lives tethered to the star that created them. But some renegade worlds wander across the Milky Way without a host. Two new analyses suggest that Jupiter-sized rogue planets are a lot less common than scientists thought. The galaxy is likely home to about 100 billion of these planets, one study shows, instead of the 200 billion proposed in 2011.

Two teams of researchers presented their findings on February 2, 2017 at a conference in Pasadena, California: one based on a recent statistical analysis and the other on observations of more than 2,600 microlensing events. These occur when a planet passes between Earth and a distant star at just the right angle to — temporarily — act like a cosmic magnifying glass. It can briefly brighten the light from the star and provide researchers with information about the size of the ‘lens’, or

planet, which might not reflect any light.

This technique is currently the only way to spot these giant rogues.

Previous estimates of the number of free-floating planets stemmed from a 2011 analysis of 10 microlensing events suggestive of renegade worlds. The authors hypothesised that there could be as many as two rogue worlds for every main-sequence star — one of the most common categories, which includes the Sun. The 2011 finding flew in the face of how many astronomers thought rogue planets formed. In a binary system where each star hosts its own planets, the gravitational force of one of the stars could disrupt the orbit of a planet and fling it out of the system. There could be similar effects in a crowded cluster of stars, where one star could eject its neighbour’s outer planet.

These and other scenarios could produce some free-floating planets, but prob-

LESS COMMON New analyses cut down the estimated number of planets unattached to a star by half.

PHOTO BY RON DANTOWITZ/CCO/NASA

ably not hundreds of billions of them, says Sarah Dodson-Robinson, an astronomer at the University of Delaware in Newark, USA. The reduced estimates have reas-

sured astronomers that their ideas of in New Zealand. The authors examined rogue planet formation aren’t so far off 474 detections of giant planets more masthe mark. sive than Jupiter, of which 10 seemed to be observations of planets that weren’t More data needed bound to stars.

Many of the suspected “rogue planets” The team’s statistical analysis suggestfrom the 2011 analysis do actually orbit a ed that the galaxy is home to roughly 200 star, says Christian Clanton, an as- billion free-floating worlds. Some tronomer at NASA Ames Research Centre researchers took issue with that interprein Moffett Field, California, USA, who was tation because it was based on so few one of the conference presenters. His sta- sightings of potential rogue planets, says tistical analysis showed that they probably Jennifer Yee, an astrophysicist at the Harcircle their star at a distance of 1.5 billion vard-Smithsonian Center for Astrokilometres or more — farther than Saturn physics in Cambridge, Massachusetts, is from the Sun. There are perhaps only USA. But the field had to wait for more half as many genuinely rogue worlds as data from better-situated telescopes to the 2011 analysis estimates, says Christian, narrow down the true number. although the planets could still number in Those observations came from the Opthe billions. tical Gravitational Lensing Experiment, The 2011 paper looked at data from the which uses a telescope in northern Chile Microlensing Observations in Astro- — a site with better weather and atmosphysics project, which uses a telescope pheric conditions for astronomy than New

Zealand. When Przemek Mroz, an astronomer at Warsaw University Observatory in Poland, and his colleagues analysed the data, they didn’t find evidence for a large population of rogue planets. Przemek — whose team is in the process of publishing its findings — points out that flaring stars, or stars whose brightness fluctuates, could mimic the microlensing effects of a renegade world.

Astronomers are looking forward to even better estimates when NASA’s Wide Field Infrared Survey Telescope launches in the mid-2020s. Once WFIRST settles into position, it will collect much more sensitive microlensing observations of planets. It will even be able to detect worlds that are smaller than Mars. The truth about rogue planets is out there, and WFIRST will help astronomers find it.

Ramin Skibba The New York Times