astronomy.i`m extremely confused?

Question

i`ve got a test and it ask how long ago did the formation of te protoplanetary disc occur and then ask for the age of earth which i know. but then say`s providing a reason,briefly explain by how much he age given in your answer (first) differs from the age of the earth.
i tough that when the protoplanetary disc occour thats when the earth was born so that would b the same age am i right???

the age of earth is 4.6 billion years that`s it lol

Answer 1

Its difficult to deal with ages of objects, whether they are protoplanetary accretion discs or planets when they are evolving objects and not formed in a brief period of time.
The word "protoplanetary" tells you that it existed BEFORE the planets. It evolved over millions of years as part of a stellar nursery nebula (thousands of light-years across) collapsed by gravitational forces. One could beg the question, "when did the accretion disc stop being a protoplanetary disc?" The planets are STILL evolving...especially the Earth. To pin a particular date on the "birth" of the Earth is impossible...for when did it stop being a protoplanet and become a planet?

In my astrophysics classes, I would define the "age of the Earth" as that of the oldest rocks of the crust. Even this is difficult to use because of the crust's constant recycling through plate tectonics. So, at what point in planetary evolution does one stop calling an evolving body a protoplanet and call it a planet? The accretion disc's oldest known material is from a class of primative meteorite known as carbonaceous chondrites...radiometric age as being about 5.5bybp. The planets are still growing to this day by accretion (every meteor adds to our planet's mass) so can we not say the accretion disk of the solar system is still extant?

You have a right to be confused! As far as this retired astrophysics professor is concerned...there is no absolute answer to the question as stated.

Answer 2

So far as I know, we do not have a good date for the formation of the protoplanetary disk, or of how quickly it condensed; obviously, it was more than 4.6 billion years ago since that is the age of the earth.

Answer 3

The protoplanetary disk was created about 4570 million years ago. That's 4.57 billion years.
http://www.lifeinuniverse.org/noflash/Earlyearth-03-03.html

The age of the Earth is probably between 4.3 billion and 4.5 billion years old.
http://pubs.usgs.gov/gip/geotime/age.html

Answer 4

The protoplanetary disc is the disc of material -- gas, rocks, dust -- from which the Earth formed. So it came first!