Gliese 581 d and Gliese 581 g
Gliese 581 d or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra.
It is the third planet discovered in the system and the fifth in order from the star.
Because of its mass, at least 5.6 times that of Earth, the planet is classified as a super-Earth. In late April 2009, new observations by the original discovery team concluded that the planet is on the outskirts of the habitable zone where liquid water may exist.
It was originally thought that Gliese 581 d orbits outside the habitable zone of its star. However, in 2009 the original discovery team revised its original estimate of the planet's orbital parameters, finding that it orbits closer to its star than originally believed. They concluded that the planet is within the habitable zone where liquid water could exist. According to Stéphane Udry, "It could be covered by a 'large and deep ocean'; it is the first serious ocean planet candidate."
Gliese 581 g is probably even better though:
Studies indicate that the planet is situated near the middle of the habitable zone of its parent star, where temperatures are neither too hot nor too cold. If it is a rocky planet, favorable atmospheric conditions could permit the presence of liquid water, a necessity for all known life, on its surface. With a mass 3.1 to 4.3 times Earth's, Gliese 581 g is considered a super-Earth, and is the planet closest in size to Earth known in a habitable zone. This makes it the most Earth-like Goldilocks planet found outside the Solar System and the exoplanet with the greatest recognized potential for harboring life.
Gliese 581 g has an orbital period of 37 days, orbiting at a distance of 0.146 AU from its parent star. It is believed to have a mass of 3.1 to 4.3 times that of the Earth and a radius of 1.3 to 2.0 times that of Earth (1.3 to 1.5 times Earth's if predominantly rocky, 1.7 to 2.0 times Earth's
if predominantly water ice). Its mass indicates that it is probably a rocky planet with a solid surface. The planet's surface gravity is expected to be in the range of 1.1 to 1.7 times Earth's,
enough to hold on to an atmosphere that is likely to be denser than Earth's.
Obviously both Jan and I texted them out there in the Gliese system:
As part of the 2009 National Science Week celebrations in Australia, Cosmos Magazine launched a website called Hello From Earth to collect messages for transmission to Gliese 581d. The maximum length of the messages was 160 characters, and they were restricted to the English language.
In total, 25,880 messages were collected from 195 countries around the world. The messages were transmitted from the DSS-43 70 m radio telescope at the Canberra Deep Space Communication Complex at Tidbinbilla, Australia on the 28th of August, 2009.
The Exo Life wager - as it stands April 30th 2007.
When will we observe the first Earth-like planet (Earth twin)
outside our own solar system?
--That is, a planet of roughly the same size, in the habitable zone around its star,
with an atmosphere, suitable for terraforming and eventually human settlements. --
See previous posts for details -
As of April 24th 07 - It almost looks like Jan has already won
Certainly, If the Exo Earth around Gliese 581 is rocky,
has an atmosphere and is suitable for human settlement
- this in is a win for Jan!
But we need confirmation on the atmosphere and its rocky'ness. And
we need furher information on whether its 2G gravity pull is within the
definition of a "twin Earth".
All of this will probably be settled in the coming years -
but sofar wager 1 looks like a win for Jan.
When will we discover the first certain signs of life outside Earth?
--A measurement on an exoplanets atmosphere that indicates life,
a fossil on Mars, bacteria in deep space or something else that
proves life outside the ecosphere of Earth.
Jan Holst Jensen:
Uncertain about detection of fossils and bacteria. But bets on
discovery of an exoplanet, with an atmosphere that indicates the
presence of life before 2020.
Bets on discovery of a fossil or a living bacteria (like structure)
or life structures (something that has the "feel" of life)
to be discovered inside our solar system and outside Earth -
within the next 50 years. I.e. before 2057.
And exoplanet with an atmosphere that indicates the presence of life
follows from one of the many interesting missions, such as Kepler, that will
be launched in the next decade. So Simon bets on discovery of an exo planet
with life before 2017.
When will we have the first signal from exo life?
-- Direct observation of exo life.
A SETI signal, or a spacecraft that observes exo life
(fossils, bacteria), or a signal (a roar or likewise) from exo life,
or footprints or .. in short - when will we have direct signs of (exo) life.
Jan Holst Jensen:
Before 2050 we will have observed an exo planet with highlevel animals comparable
Thinks there will be a discovery of some kind of extraterrestrial life
within the solar system within the next 50 years. Probably very primitive
though. Direct discovery of life outside the solar system will take new technologies
in rocket design. And apparently rockettechnologies aren't proceeding that fast.
So the bet here is as bad as a 100 - 150 years from now - which is really just to say in some very distant future.
The hope rest on SETI signals.
We will have a SETI signal within the next 50 years. So, before 2057 we will have
our signal from exo life.
When will we have our first signal from intelligent exo life?
I.e. exo life complex enough to
be on par with some of the intelligent species we have here on Earth.
Jan Holst Jensen:
Will be observed before 5060 - or never :-)
Obviously, the Fermi paradox still reigns - and could leave one
believing that we will never find intelligent extraterrestrtial life -
However, it just takes one brilliant new theory on how signals
are sent accross deep space to suddenly have a whole new approach.
So with advances in signal capabilities and general physics knowledge we
will come to explore whole new ways of communicating. Eventually, we
will tumble on a SETI signal- And it will happen before
revised April 30th 2007.
Labels: exo life, extra terrestrial life, First SETI signal
Gliese 581 exo planet - WAUV - a major step towards a new Earth
In a finding that if confirmed could stand as a landmark in history, astronomers have reported discovering the most Earth-like planet outside our Solar System to date: a world that may have liquid oceans and thus life......
Located only 20.5 light-years away - Gliese 581 is among the 100 closest stars. So close, that we could consider sending a mission towards Gliese within this century!
Lets recapitulate what this blog is all about: A wager on when we will find the first true exo Earth.
We had talked about this for years before 2002 - but 5 years ago we formalized the bet to: When will we find Earth 2? And this blog was initialized - I.e.:
[The planet we are talking about must not only look like Earth in size and color - we think it must have an atmosphere,
perhaps even a diverse landscape where life can find many different
niche's. In some distant future it should be possible for
humans to settle the place. It short, it should be VERY Earthlike.
Obviously, all other kinds of planets will also be extremely
interesting, but here we are talking about Earth II. ]
It was further understood that the Exo Earth should be within the habitable zone of its star. Gravity should be Earth like - We didn't have to many details on what the atmosphere should be like - but the planet should have an atmosphere - and it should potentially be possible to terraform that atmosphere in order for people to be able to settle the planet. The planet should also have a "home" feeling - i.e. look like Earth- where it almost follows from this that there should be liquid water on the planet (but again, we left if for the the future to settle the exact details).
And we had:
Jan Holst Jensen: - The Kepler Mission will find such a planet more or less immediately after launch. That is in 2007 - 2008.
Simon Laub: Was a little bit more cautious, ..So he is betting on 2012.
The interesting date is the confirmation date. But we are ok with a little wiggle-room for a debate on whether we should instead be talking discovery date. Discovery date is allowed - if the discovery
has all the hallmarks of a confirmation as well.
Fast forward to April 24th 2007...
Gliese's planet is in the habitable zone of the Star!
The red dwarf Gliese 581, is smaller and colder than the Sun – and thus less luminous – the planet lies in the habitable zone, the region around a star where water could be liquid!
Glies's planets gravity would be twice that of Earth.
Which then makes it open to interpretation, whether that is something humans could settle. But for now it seems ok.
Moreover, the planet around Gliese's radius is estimated to only 1.5 times the Earth’s radius, and models predict that the planet should be either rocky – like our Earth – or covered with oceans,” he said.
Obviously, we need confirmation on all of this. But so far this is very promising.
With water and an atmosphere on Gliesse's Earth like planet - a nice blue feel to it - Jan is the winner of our wager.
Long before the Kepler mission even made it to liftoff...
But we need more details. The wager is not settled yet!
The Gliese story so far - April 24th 2007:
In a finding that if confirmed could stand as a landmark in history, astronomers have reported discovering the most Earth-like planet outside our Solar System to date: a world that may have liquid oceans and thus life.
Swiss, French and Portuguese scientists found the body, estimated as 50 percent wider than our Earth, orbiting a so-called red dwarf star relatively close to Earth. The star is thought to harbor two other planets also.
The newfound exoplanet—as astronomers call planets around stars other than the Sun—would be the smallest such body ever reported.
Nonetheless, the object is estimated to weigh as much as five Earths, partly thanks to its greater width. For the same reason, it would have more than twice Earth’s surface area. Historically, only large exoplanets lend themselves to human detection, though that is changing.
Other curious features of the newfound planet are that gravity at its surface would be around twice as strong as on Earth; and its year is just 13 Earth days long, as it completes one orbit about its sun in that time.
It’s 14 times closer to its star than we are from our Sun, researchers said. But since its host star, the red dwarf Gliese 581, is smaller and cooler than the Sun, the planet nevertheless would lie in its habitable zone—the region around a star with suitable temperatures for liquid water.
Average temperatures on this “super-Earth” lie between 0 and 40 degrees Celsius (32 to 104 degrees Fahrenheit), “and water would thus be liquid,” said Stéphane Udry of Switzerland’s Geneva Observatory, lead author of a paper reporting the result. “Models predict that the planet should be either rocky—like our Earth—or covered with oceans,” he added.
Liquid water is critical to life as we know it,” noted Xavier Delfosse, a member of the team from Grenoble University, France.
“Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X.”
The host star, Gliese 581, is among the 100 closest stars to us, lying 20.5 light-years away in the constellation Libra (“the Scales.”) A light-year is the distance light travels in a year.
Gliese 581 has one third the mass of our Sun. Such small stars, called red dwarfs, are at least 50 times fainter than the Sun and are believed to be the most common stars in our galaxy. Among the 100 closest stars to the Sun, 80 belong to this class.
“Red dwarfs are ideal targets for the search for such planets because they emit less light, and the habitable zone is thus much closer to them than it is around the Sun,” said Xavier Bonfils, a co-researcher from Lisbon University. Planets near a star are easier to detect because their gravitational pull affects the parent star noticeably, inducing something of a wiggling motion.
Red dwarfs are also expected to live extraordinarily long because they burn fuel slowly. A red dwarf one-third the Sun’s mass, like Gliese 581, would typically shine for some 130 billion years, outliving the Sun by thirteen times. That might relieve at least one source of stress for any inhabitants of a red dwarf system. We on Earth are already halfway through the Sun’s lifetime, though much time remains.
Two years ago, Udry and his team found another planet around Gliese 581, estimated to weigh as much as 15 Earths—about as much as Neptune—and orbiting the star in 5.4 days.
At the time, the astronomers had already noted hints of another planet, Udry and colleagues said. They thus took new measurements and found the new “super-Earth,” as well as a likely third planet weighing eight Earths and orbiting in 84 days. The findings have been submitted to the research journal Astronomy and Astrophysics, the scientists said.
The find was possible thanks to an instrument known as a spectrograph on the European Southern Observatory’s 3.6-meter telescope at La Silla, Chile, according to the group. The instrument, called the High Accuracy Radial Velocity for Planetary Searcher, is touted as one of the most successful tools for detecting exoplanets to date.
The instrument measured wiggles in the star’s motion corresponding to velocity changes of just two to three meters per second—the speed of a brisk walk, according to the Geneva Observatory’s Michel Mayor, principal investigator for the instrument. Given the results so far, “Earth-mass planets around red dwarfs are within reach” of discovery, he predicted.
An international team of astronomers from Switzerland, France and Portugal have discovered the most Earth-like planet outside our Solar System to date.
The planet has a radius only 50 percent larger than Earth and is very likely to contain liquid water on its surface.
The research team used the European Southern Observatory’s (ESO’s) 3.6-m telescope to discover the super-Earth, which has a mass about five times that of the Earth and orbits a red dwarf already known to harbour a Neptune-mass planet.
Astronomers believe there is a strong possibility in the presence of a third planet with a mass about eight times that of the Earth in the system.
However, unlike our Earth, this planet takes only 13 days to complete one orbit round its star. It is also 14 times closer to its star than the Earth is from the Sun.
However, since its host star, the red dwarf Gliese 581, is smaller and colder than the Sun – and thus less luminous – the planet lies in the habitable zone, the region around a star where water could be liquid!
“We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid,” said Stéphane Udry from the Geneva Observatory, Switzerland and lead-author of the paper in the journal Astronomy and Astrophysics.
“Moreover, its radius should be only 1.5 times the Earth’s radius, and models predict that the planet should be either rocky – like our Earth – or covered with oceans,” he said.
“Liquid water is critical to life as we know it and because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X,” added Xavier Delfosse, a member of the team from Grenoble University, France.
According to the research team, the host star, Gliese 581, is among the 100 closest stars to us, located only 20.5 light-years away in the constellation Libra (“the Scales”).
The star has a mass only one third that of the Sun. Such red dwarfs are at least 50 times intrinsically fainter than the Sun and are the most common stars in our Galaxy. Among the 100 closest stars to the Sun, 80 belong to this class.
“Red dwarfs are ideal targets for the search for such planets because they emit less light, and the habitable zone is thus much closer to them than it is around the Sun. Any planets that lie in this zone are more easily detected with the radial-velocity method, the most successful in detecting exoplanets,” said Xavier Bonfils, a co-worker from Lisbon University.
Labels: a new Earth, exo Earth, Gliese 581
Getting closer - exo planet with water
We are getting closer to that Exo Earth ... all the time.
Astronomers have detected water in the atmosphere of a planet outside our solar system for the first time.
The finding, to be detailed in an upcoming issue of Astrophysical Journal, confirms previous theories that say water vapor should be present in the atmospheres of nearly all the known extrasolar planets. Even hot Jupiters, gaseous planets that orbit closer to their stars than Mercury to our Sun, are thought to have water.
The discovery, announced today, means one of the most crucial elements for life as we know it can exist around planets orbiting other stars.
“We know that water vapor exists in the atmospheres of one extrasolar planet and there is good reason to believe that other extrasolar planets contain water vapor,” said Travis Barman, an astronomer at the Lowell Observatory in Arizona who made the discovery.
HD209458b is a world well-known among planet hunters. In 1999, it became the first planet to be directly observed around a normal star outside our solar system and, a few years later, was the first exoplanet confirmed to have oxygen and carbon in its atmosphere.
HD209458b is separated from its star by only about 4 million miles (7 million kilometers)—about 100 times closer than Jupiter is to our sun—and is so hot scientists think about it is losing about 10,000 tons of material every second as vented gas.
"Water actually survives over a broad range of temperatures," Barman explained. "It would need to get quite a bit hotter to completely break the water molecules apart."
Using a combination of previously published Hubble Space Telescope measurements and new theoretical models, Barman found strong evidence for water absorption in the atmosphere of the extrasolar planet HD209458b.
Barman took advantage of the fact that HD209458b is a so-called “transiting planet,” meaning it passes directly in front of its star as seen from Earth. It transits every three-and-a-half days.
When this happens, water vapor in the planet’s atmosphere causes the planet to appear slightly larger in the infrared part of the starlight than in the visible portion.
Barman found the water signature after applying new theoretical models he developed to visible and infrared Hubble data collected by Harvard student Heather Knutson last year, which measured the perceived size of the planet over a broad range of wavelengths.
Other scientist wonder why many exo planets don't have
water - wondering whether they have been to Earth centric.....
Scientists taking their first "sniffs of air" from planets outside our solar system are baffled by what they didn't find: water.
One of the more basic assumptions of astronomy is that the two distant, hot gaseous planets they examined must contain water in their atmospheres. The two suns the planets orbit closely have hydrogen and oxygen, the stable building blocks of water. These planets' atmospheres — examined for the first time using light spectra to determine the air's chemical composition — are supposed to be made up of the same thing, good old H2O.
But when two different teams of astronomers used NASA's Spitzer Space Telescope for this new type of extrasolar planet research, they both came up dry, according to research published in Thursday's edition of Nature and the online version of the Astrophysical Journal Letters.
The study of one planet found hints of fine silicate-particle clouds. Research on the other planet found no chemical fingerprints for any of the molecules scientists were seeking.
Approach too ‘Earth-centric’
"We had expected this tremendous signature of water ... and it wasn't there," said the study leader for one team, Carl Grillmair of the California Institute of Technology and Spitzer Science Center. "The very fact that we've been surprised here is a wake-up call. We obviously need to do some more work."
Grillmair's colleague, Harvard astronomy professor David Charbonneau, said these surprising "sniffs of air from an alien world" tell astronomers not to be so Earth-centric in thinking about other planets.
"These are very different beasts. These are unlike any other planets in the solar system," Charbonneau said. "We're limited by our imagination in thinking about the different avenues that these atmospheres take place in."
Our own solar system has two planets without water in the atmosphere, Grillmair noted: Mercury, which doesn't have an atmosphere, and Venus, which is a different type of planet from the huge gaseous ones that would be expected to have the components of water in the air.
Water may be hiding, scientists suggest
So far, scientists have found 213 planets outside our solar system, but only 14 have orbits that make it possible for this type of study; only eight or nine of those are close enough to see. Grillmair's team studied the closest, which goes by the catchy name HD 189733b. It's a mere 360 trillion miles from Earth in the constellation Vulpecula. The other planet, HD209458b, is about 900 trillion miles away in the constellation Pegasus, and it's the one with the strange silicate cloud.
So where'd the water go?
Maybe it's hiding, scientists suggest. The water could be under dust clouds, or all the airborne water molecules have the same temperature, making it impossible to see using an infrared spectrograph. Or maybe it's just not there and astronomers have to go back to the drawing board when it comes to these alien planets.
The other finding on the more distant of the two planets seems to indicate that the atmosphere is full of silicon-oxygen compounds, said study lead author L. Jeremy Richardson of NASA's Goddard Space Flight Center.
"They'd be like dust grains and they would form clouds," Richardson said. And that cloud of silicates could be blocking the space telescope from measuring lower-lying water, he and other scientists said.
Labels: exo Earth, exo planet, water discovered in another solar system
A year later - the bet is still on!
A year later the bet is still on! We don't have a winner yet. But we are getting closer!
Now we are down to 5 times the size of Earth I. Still no Atmosphere and still way to cold - but improving all the time!
WASHINGTON (Reuters) -- A new planet-hunting technique has detected the most Earth-like planet yet around a star other than our sun, raising hopes of finding a space rock that might support life, astronomers reported on Wednesday.
"This is an important breakthrough in the quest to answer the question 'Are we alone?'" said Michael Turner of the National Science Foundation.
"The team has discovered the most Earth-like planet yet, and more importantly, has demonstrated the power of a new technique that is sensitive to detecting habitable planets," Turner said in a statement.
In the last decade, astronomers have detected more than 160 planets orbiting stars outside our solar system. The vast majority of these have been gas giant planets like Jupiter, which are hostile to life as it is known on Earth.
But an international team has detected a cold planet about 5-1/2 times more massive than Earth -- still small enough to be considered Earth-like -- orbiting a star about 20,000 light-years away in the constellation Sagittarius (The Archer), close to the center of the Milky Way.
A light-year is about 6 trillion miles (10 trillion km), the distance light travels in a year.
To find this new planet -- named OGLE-2005-BLG-390Lb -- the team used a technique called gravitational microlensing.
This method uses a network of telescopes to watch for changes in light coming from distant stars. If another star passes between a distant star and a telescope on Earth, the gravity of the intervening star acts like a lens and magnifies the incoming light.
When a planet is circling the closer star, the planet's gravity can add its own signature to the light, the scientists said in research being published in the current edition of the journal Nature.
This kind of light signature was observed on July 11 by a group of telescopes participating in a project known as OGLE, short for Optical Gravitational Lensing Experiment, which sees more than 500 microlensing events each year.
The OGLE observation alerted the planet-hunting telescopes of PLANET (Probing Lensing Anomalies NETwork) and Robonet, which made further observations and on August 10 detected the presence of a previously unknown planet.
Astronomers have been discovering so-called extrasolar planets for the last decade, but most have used a method that looks for a characteristic wobble in stars caused by the unseen planets that orbit around them. This technique has been successful in finding Jupiter-type planets but few with Earth's mass.
However, the microlensing technique may hold promise for detecting more planets like our own, in the habitable zone neither too torridly close nor frigidly far from the stars they orbit, said David Bennett of the University of Notre Dame in Indiana, a member of the PLANET team.
"The main advantage (of microlensing) is the signals for low-mass planets: they're not weak signals, they're just rare," Bennett said by telephone. "If there happens to be a good alignment between a foreground star with its planet and the background source star, then you're able to detect that planet."
Betting on a new Earth!
When, if ever, will humans discover another Earth?
That has been the subject of many a good discussion.
Here in Denmark my friend Jan Holst Jensen and I have discussed
it many times.
Now however, the time has come where we commit our
prediction to this blog - for some future laughs!
The planet we are talking about must not only look like Earth in size and color - we think it must have an atmosphere,
perhaps even a diverse landscape where life can find many different
niche's. In some distant future it should be possible for
humans to settle the place. It short, it should be VERY Earthlike.
Obviously, all other kinds of planets will also be extremely
interesting, but here we are talking about Earth II.
So, when are we going to see it? After endless discussions -
Finally, in 2002 we made a bet on it (the winner gets the prestige of being right). The projects that began to emerge at that time made us confident that it will be possible to settle the question in our lifetime.
So here we go:
Jan Holst Jensen: - The Kepler Mission will find such a planet more or less immediately after launch. That is in 2007 - 2008. And Kepler might not even be necessary, some lone astronomer might beat Kepler... So Jan is betting on 2009 (Where Kepler have made three observations
of Earth II).
Simon Laub: Was a little bit more cautious, even though he certainly shares the enthusiasm. Simon thinks you will have to monitor a system for some time to be sure - and that the task will be a little more
difficult than just picking a winner immediately after the Kepler launch.
So he is betting on 2012.
We both thinks that the Terrestrial Planet Finder mission will be necessary to be absolutely sure - but both thinks that the smoking gun will be there before this mission. Which allows us to stand up for these early dates.
The bet is now committed to this blog. everybody should feel
free to comment or give their own bet to this blog. We should have
a winner in less than 20 years ! :-)
So far the facts are:
Kepler is planned for launch in the fall of 2007, it will monitor 100,000 stars similar to our sun for four years. The results will be extremely important either way. If Kepler detects many habitable, Earth-size planets, it could mean the universe is full of life.
When a planet passes in front of its parent star, as seen by us, it blocks a small fraction of the light from that star. If the dimming is truly caused by a planet, then the transits must be repeatable. Measuring three transits all with a consistent period, duration and change in brightness provides a rigorous method for discovering and confirming planets - planets even smaller than the Earth.
Kepler will help answer the questions:
What fraction of stars have planetary systems?
How many planets are there in a typical system, and
What are their masses and distances from the central star?
How do these characteristics depend on the mass of the star, its age and whether it has a binary companion?"
What will Kepler find? That's the exciting part, because we don't know. Borucki has estimated that if Earth-sized planets are common, the mission will uncover roughly 50 of them in orbits comparable to our own. Thousands of closer-in and/or larger planets could be found. Even bulky moons around some of the Jupiter-sized planets discovered with the wobble technique might block enough starlight to make their presence known. All such worlds are potential abodes for life.
After Kepler the Terrestrial Planet Finder will continue its work
The Terrestrial Planet Finder (TPF) will use a small collection of high sensitivity telescopes (probably 4 large 3.5-meter telescopes). It will measure the temperature, size, and the orbital parameters of planets as small as our Earth in the habitable zones of distant solar systems. Also, TPF's spectroscopy will allow atmospheric chemists and biologists to use the relative amounts of gases like carbon dioxide, water vapor, methane and ozone to find whether a planet might support life. Launch is anticipated between 2012-2015.
Jan Holst Jensen (Cph, Denmark)
Simon Laub (Aarhus, Denmark)
The ET message is in our genes!
Although many planets have been discovered outside the solar system, so far none of them looks anything like our planets.
Typically, these planets are are much heavier than Jupiter, and
most are so-called "hot jupitors" that orbit closer to their star than does the Earth.
A new Earth has not been found yet. And certainly
not a new Earth with aliens beaming radiosignals to us.
Still, all the new exo-planets tend to make you optimistic.
That ET signal can't be far away? Or?
For more than 40 years, mankind has been
using radio telescopes to pick up signals from
And so far the silence has been deafening......
Which might not be that much of a surprise.
After all, Alien civilisations could be million of years ahead
of us. Either they should continue to send signals
in our direction for aeons, hoping that
we would one day build a radio telescope. Or they
have only transmitted sporadically, in which case we
have an about zero percent chance of tuning in
at the right time.
So, if the ETs really want to contact us it would be
much better for them to leave some kind of superstructure
on our planet or in its vicinity - which would then
"phone home", when we are evolved enough to be interesting
(2001, A Space Odyssey).
Certainly a nice idea, but it also
has its problems - such artefacts on a planet surface
might be overlooked (by the dummies) or eroded away over the aeons.
A better solution would be to have messages inserted
into something that is small, cheap, self-repairing and self-replicating.
And which is always close to the species (dummies)
you want to contact. Something that keep copying the information
over immense durations and despite whatever unforseen environmental hazards might pop up.
And we have such things - they are called cells!
So, according to Apollo astronaut Rusty Schweickard the place to
look for messages from ET is in DNA.
Obviously, there are mutations in the DNA - so ET
would probably incorporate messages in parts that are highly conserved.
Unfortunately, such parts are normally essential coding parts
that control the most vital parts of an organism.
Inserting messages in non protein coding parts - socalled "junk DNA" - have a better chances of being not harmful for the organism.
But such parts are also likely to accumulate lots of mutations
over the aeons, that would destroy the message?
Fortunately, highly conserved sequences of "junk DNA"
have recently been discovered.
So, that could be a nice spot to locate the primer for
how to go online to the Encyclopedia Galactica.