Use full text to rank results higher if they are "closer hit"

Hello,

I think it's easier to explain my question with example code:

------------------------
CREATE TABLE t ( s VARCHAR );
CREATE TABLE

INSERT INTO t VALUES ('hello'), ('hello world');
INSERT 0 2

SELECT * FROM t;
s
-------------
hello
hello world
(2 rows)

SELECT s, ts_rank(vector, query) AS rank
FROM t, to_tsvector(s) vector, to_tsquery('hello') query
WHERE query @@ vector;
s | rank
-------------+-----------
hello | 0.0607927
hello world | 0.0607927
(2 rows)
------------------------

Here both 'hello' and 'hello world' are ranked equally highly when
searching with 'hello'. What I'm wondering is, is there a way within
postgres to have it match higher to just 'hello' than 'hello world'?
I.e. something like it slightly down-weights extraneous terms? Of course
in general I don't know the query or the field strings ahead of time.

Thanks for any help!

Cheers,
Thomas